Islamic state could very well cause world war 3, but it wouldn't be direct, if you see what I mean.
It is far more likely that a state reaction to something IS does creates a wider conflict: say, IS kills another group of Egyptians, Egypt orders an large attack on IS positions, something goes wrong, the bombs end up being dropped on Cyprus (the Republic is an EU state) and wipes out a British base, this creates an enormous diplomatic incident ... even more so when it is discovered that the Egyptians were using Turkish guidance systems.
Then the narrative of a "mistake" comes to be about the next chapter in the Eastern Med energy wars and control over gas rights, which is one of the readings of IS action anyway (that they are a proxy force designed to eradicate Russian control over the European gas markets in favour of Qatar).
And it all becomes a really big mess that pulls in Russia, the EU, the US, the Chinese (who are already pissed off that the war in the Ukraine has screwed up their food planning programme) ...
... and, at this point, I go and live in the forest and change my name to Sparkly Unicorn Hazelnut Hat.