Well, I think it's very possible.
I'll discuss the possibility of this particular war later in the post, but look at the other two World Wars.

World War 1-
Sparks-
Rising power and military buildup of Prussia, unification of Italy, various smaller wars, clever politicians Bismarck and Cavour.
Detonator-
Balkan Powder Keg
Sides-
Allied Powers(Triple Entente+Italy+Ottoman Empire)
Central Powers(Triple Alliance-Italy)

The Balkans were a region heavily influenced by larger, surrounding nations. Massive differences in culture and influence started the war, essentially, and the entirety of Europe was plunged into a war.
Sidenotes-
The Ottoman Empire was dealing with a small revolt of its own at the time, and Russia was being overhauled.
Powers Involved-
Russia, Great Britain, France, Austria-Hungary, Italy, Ottoman Empire/Turkey, United States(late war)
World War 2-
Alright, this is the one that gets interesting.
'sparks'
Lack of recovery from World War 1's Economic strains in many countries.
Deep rooted hatred between France and Prussia(Germany)
Rise of Fascism in powerful European Nations, such as Italy and Germany.
Imperialism in a newly modernized Japan
Anti semitism towards Jews
Radical Muslim Governments in Middle East who supported Hitler's goals to eradicate the Jews
Powers Involved-
Russia, Great Britain, France, Germany, Italy(much larger player than last war), Japan, United States(late war)
Detonator-
Formation of the Axis Powers and German invasion of Poland.

Now then, here's modern times.
Sparks-
Russian aggresion towards Ukraine
Anti Semitism in Europe
I.S.I.S. in nearly every country and US state as well as being based in the Middle East.

The detonator? Very likely a Russian invasion of Ukraine. The United States wouldn't get involved until later in the war, as Obama would never be strong enough to get in the way of other powers.

Sadly, I completely agree that this is how World War 3 will start. Also, you forgot about the North Korea issue. I personally think the war will start with a nuclear missile going off. (*cough* North Korea *cough*) But, that's just me. I hope this doesn't happen in my or any of our lifetimes, even though it seems like it's going to happen very soon. I hope that everyone in my and other countries will remain safe without war sparking.

LunarDarkness2 : You're right, I did forget to mention North Korea, and Iran has similar military buildup as well. But yeah, you do have a point, and thanks for posting too, that'll get more attention to the thread and more discussion as well, which leads to input and interesting coversations.

While I could potentially see these escalating to military conflicts, I don't see them getting much further.

On a different note, hopefully in the coming decade we perfect some sort of laser system that would stop nuclear/other missiles.

EideticMemory : Actually, you're pretty close with that laser system. The United States a,ready has one in Iran to deal with small craft and incoming missiles.
And way to be a buzzkill. (Kidding, kidding, I like your posts, they're almost always good)

World War 3 is not going to happen for at least another 30 years, the situation isn't exceedingly unstable, and even potentially dangerous countries don't have the funds to spend on a large scale war, maybe save for Russia, but even then I don't see the Russia-Ukraine conflict escalating to world war proportions.

Besides the United Nations will make sure to lessen its impact or prevent it from happening to begin with. Mind you the UN did not exist when WWII  happened

World War 3?  I suppose.  If that's what you want to call it.  The next "big" war coming is (mainly) Russia in alliance with other Asian countries (Turkey, Libya, Iran, Sudan, Syria etc.) against Israel.  The outcome for Russia and partners will not be pretty I'm afraid.

EideticMemory :  Israel actually already have such a system.  Not a STAR WARS type anti-missile laser beam, but they do have an anti-missile system that worked pretty well against HAMAS.

Myelin : But we did have a worldwide 'alliance',' The League of Nations, which, despite being weaker, still had some say.
Lao, another factor involving ISIS with the beginning of World a War Three, which you reminded me of.
Russia most certainly does not have the funds to start a large scale war. One of these reasons is cheap, Black Market oil coming in from the Islamic State simply to crush the Russian economy. Russia's much more expensive barrels are being practically ignored by even gray area business to get higher profits on oil.
As for the funding look at a World War Two. Did anyone have the funding to go into a major world war? No. But it jump-started the economies of many depression-struck nations by giving so many people jobs.

SoL@R : What you're referring to is the Iron Dome system, it takes out about 80% of incoming rockets and mortars, but unfortunatly is very, very expensive.

Also. Other factors-North Korea, but they're always there, so...you know.
Another thing is that Asian countries, inculding Japan, are building up their armies, and so is Germany(which has never turned out well, despite my love for the country)
Also conflict between NATO and Russia, just an FYI.