In Pokemon Ruby 3 in 1 you can catch all three starters, Treecko, Torchic and Mudkip and here is a little bit about the three starters to better sway your opinion on if you want to catch them or not. 

Treecko- grass type Pokemon, it's ability is overgrow which boosts up grass type moves a pinch during battle.

Torchic- fire type Pokemon, its ability is blaze which boosts up fire type moves in battle

Mudkip- water type Pokemon, it's ability is  torrent which boost water type moves during battle
Now the starters all have three evolution's and here is a little bit about their evolution's and possible move set's.

Treecko- It's second evolution(evolves at 16) is Grovyle and his final third evolution(evolves at level 36)  is Sceptile. It is a grass type throughout all of its evolution's and here is some of the possible moves it can learn throughout its evolution stages by level up and Hm's/TM's, Pound, leer,absorb, quick attack, absorb, furry cutter, pursuit,screech,leaf blade, agility,slam, detect false swipe,cut strength, aerial ace,protect,brick break,solar beam,hyper beam,giga drain(note these are just a few moves it can learn not all of them are listed here)

Torchic-It's second evolution(evolves at level 16) is Combusken and it's third and final evolution(evolves at 36) is Blaziken. Torchics second & third evolution's, Blaziken & Combusken are both fire and fighting types, and throughout all of its evolution's  here is some of the possible moves it can learn throughout its evolution stages by level up and Hm's/TM's,ember,peck, double kick, sky upper cut, fire punch, cut strength, solar beam, brick break, flamethrower,aerial ace, overheat, fire blast, rock smash, protect(note these are just a few moves it can learn not all of them are listed here)

Mudkip-It's second evolution(evolves at level 16) is Marshtomp and it's third and final evolution(evolves at level 36) is Swampert. Mudkip's second & third evolution's, Marshtomp and Swampert are both water and ground types, and throughout all of its evolution's  here is some of the possible moves it can learn throughout its evolution stages by level up and Hm's/TM's,water gun, mud slap,bide,mud shot, muddy water, protect, earthquake, hammer arm,surf, strength, waterfall,dive,hyper beam protect, ice beam, blizzard, brick break (note these are just a few moves it can learn not all of them are listed here)
Now we move on to the gyms in order starting with the first, where I will list the leader and what Pokemon she uses as well as the Pokemon's level and type.

Gym #1 Roxanne- Geodude level 14 rock type,Nosspass level 15 rock type.
(For those who don't know Water, grass and fighting types beat Rock types so I would suggest using one of those three types)
Gym#2 Brawly- Machop level 17 fighting type, and Makuhita level 18 fighting type.
(For those who don't know physic, and flying types beat fighting types so I would suggest using one of those two types)
Gym#3 Wattson- Magnemite level 22 electric steel type, Voltorb level 20 electric type, Magneton level 23 electric steel type.
(For those who don't know grass, and ground and fire types beat electric and steel types so I would suggest using one of those three types)
Gym#4 Flannery- Slugma level 26 fire type, Slugma level 26 fire type, Torkoal level 28 fire type.
(For those who don't know water, and ground and rock types beat fire types so I would suggest using one of those three types)
Gym#5 Norman- Slaking level 28 normal type, Vigoroth level 30 normal type, Slaking level 31 normal type.
(For those who don't know fighting, and generally any type beats normal  types so I would suggest a fighting for a advantage but any type would/will do)
Gym#6 Winona- Swellow level 31 normal and flying type, Pelipper level 30 water and flying type, Skarmory level 32 steel and flying type, Altaria level 33 dragon and flying type.
(For those who don't know electric and ice types beats flying types so I would suggest a electric for a advantage but for the duel types, Swellow  I would use electric or fighting, for Pelipper I would use grass or electric, for Skarmory I would use electric, fire or ice types, and for Altaria I would use ice or electric, or a dragon type)
Gym#7 Liza and Tate ( the only duel gym leaders in Pokemon ever also this will be a double battle)-  Solrock level 42 physic and rock type, Lunatune rock and physic type
(For those who don't know Dark and ghost as well as water grass and fighting types beat rock and physic  types so I would suggest any combination of the types listed to win)
Gym#8 Wallace- Luvdisc level 42 water type, Whiscash level 42 water and ground type, Sealeo level 44 water and ice type, Seaking level 42 water type, Milotic level 45 water type.
(For those who don't know  grass and electric  types beat water  types so I would suggest any combination of the types listed to win)

The Elite Four
Now begins the elite four part of my guide, here I will list the elite four members in order from 1-4 and then the champion and the Pokemon, the Pokemon types and level and moves they use.

#1 Elite Four member Sidney-
Types of Pokemon he uses, Dark Type
Level 46 Mightyena. Type: Dark
Moves:
Sand-Attack
Roar
Take Down
Crunch
(suggestion for beating Mightyena use a ghost type)
Level 48: Shiftry  Type: Grass/Dark
Moves:
Fake Out
Swagger
Double Team
Extrasensory
(suggestion for beating Shiftry use a fire or flying or ghost type)
Level 46: Cacturne. Type: Grass/Dark
Moves:
Cotton Spore
Needle Arm
Faint Attack
Leech Seed
(Suggestion for beating Cacturne use fire or flying type)
Level 48: Sharpedo. Type: Water/Dark
Moves:
Swagger
Slash
Surf
Crunch
(suggestion for beating Sharpedo use a electric or grass type or ghost)
Level 49: Absol. Type: Dark
Moves:
Snatch
Aerial Ace
Swords Dance
Slash
(suggestion for beating Absol use a ghost or fighting type)
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#2 Elite four member Phoebe-
Level 48: Dusclops. Type: Ghost
Moves:
Future Sight
Curse
Confuse Ray
Shadow Punch
(suggestion for beating Dusclops use a dark or physic type)
Level 49: Banette. Type: Ghost
Moves:
Spite
Will-O-Wisp
Shadow Ball
Faint Attack
(suggestion for beating Banette use a dark or physic type)
Level 50: Sableye. Type: Ghost/Dark
Moves:
Psychic
Shadow Ball
Faint Attack
Attract
(suggestion for beating Sableye use generally any type that isn't physic)
Level 49: Banette. Type: Ghost
Moves:
Psychic
Toxic
Shadow Ball
Skill Swap
(suggestion for beating Banette use a dark or physic type)
Level 51: Dusclops. Type: Ghost
Moves:
Ice Beam
Shadow Ball
Confuse Ray
Earthquake
(suggestion for beating Dusclops use a dark or physic type)
____________________________________________________________________________________________________________________
#3 Elite Four member Glacia-
Level 50: Glalie. Type: Ice
Moves:
Hail
Light Screen
Ice Beam
Crunch
(suggestion for beating Glalie use grass, electric, fighting or fire type)
Level 50: Sealeo. Type: Ice/Water
Moves:
Surf
Ice Ball
Hail
Body Slam
(Suggestion for beating Sealeo use electric or grass or fighting or fire type)
Level 52: Sealeo. Type: Ice/Water
Moves:
Hail
Blizzard
Attract
Dive
(Suggestion for beating Sealeo use electric or grass or fighting or fire type)
Level 52: Glalie. Type: Ice
Moves:
Hail
Shadow Ball
Ice Beam
Crunch
(suggestion for beating Glalie use grass, electric ,fighting or fire type)
Level 53: Walrein. Type: Ice/Water
Moves:
Surf
Body Slam
Blizzard
Sheer Cold
(suggestion for beating Walrein use grass, electric, fighting or fire type )
____________________________________________________________________________________________________________________
#4 Elite Four member Drake-
( Just going to be blunt use ice and water types to defeat all Pokemon listed)
Level 52: Shelgon. Type: Dragon
Moves:
Rock Tomb
Dragon Claw
Protect
Crunch
Level 54: Altaria. Type: Dragon/Flying
Moves:
Dragon Dance
DragonBreath
Take Down
Refresh
Level 53: Flygon. Type: Dragon/Ground
Moves:
SandStorm
Dig
DragonBreath
Fly Level 53: Flygon. Type: Dragon/Ground
Moves:
DragonBreath
Flamethrower
Sand-Attack
Crunch
Level 55: Salamence. Type: Dragon/Flying
Moves:
Dragon Claw
Crunch
Flamethrower
Fly
___________________________________________________________________________________________________________________
The Champion Steven-
Level 57: Skarmory Type: Steel/Flying
Moves:
Toxic
Steel Wing
Aerial Ace
Spikes
( Suggestion I would use fire, ice or electric type)
Level 55: Claydol Type: Ground/Psychic
Moves:
Light Screen
AncientPower
Reflect
Earthquake
(suggestion I would use water,dark ,ice or grass or ghost type)
Level 56: Aggron. Type: Steel/Rock
Moves:
Earthquake
Thunder
Dragon Claw
SolarBeam
(suggestion I would use a electric,water,grass or fighting type)
Level 56: Cradily. Type: Rock/Grass
Moves:
Confuse Ray
Sludge Bomb
AncientPower
Giga Drain
(suggestion I would use a fire,fighting physic type)
Level 56: Armaldo. Type: Rock/Bug
Moves:
Slash
AncientPower
Aerial Ace
Water Pulse
(suggestion I would use a fire.physic, water or fighting type )
Level 58: Metagross. Type: Steel/Psychic
Moves:
Earthquake
Meteor Mash
Psychic
Hyper Beam
(suggestion I would use a fire, ghost or fighting type)
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I only skimmed through this since I have played through this game (Well, the original Ruby) around 20 times and I know all of that info already. You did a pretty good job giving information on the starters, gym leaders, and elite four. You gave a basic strategy guide, which is helpful. Nice job.

Excellent work on this walkthrough, you provide a lot of essential and vital information that is key to doing well in this game. I think you did a great job of being thorough and explaining the game and breaking it down. I am pretty sure I replied to your other recent walkthrough as well which looked pretty similar. 

You cover a lot of the basics and then you go into greater detail later on in your guide here which is good. I like how you suggest in parentheses for those who don't know an effective type, you listed a good type that would be effective. I think that this will be really helpful for someone else who decides to play the game. Nice job on this guide man, you are doing great work and your CP is shooting up like crazy. Keep up the awesome work and I find this guide really helpful.