I'd go with gunpowder. The typewriter was nothing more than an upgrade to the linotype machines (now, the original Gutenberg printing press was more important an invention than any of the three, possibly the most important invention of the modern world). The steam engine is harder to dismiss, as it revolutionized transport, but without the capture and holding of territory that gunpowder made possible, steam engines would lack the fuel to operate, not to mention the idle time needed to maintain them and find better applications for them than the "hey, look at this thing spin!" the Greeks put them to.
So, why gunpodwer? Well, it's more for the "gun" than for the "powder," really. Black powder was used in fireworks and occasional bombs by Oriental empires since long before the Europeans even invented the poleaxe. However, the Europeans were the ones who came up with the idea to load it into a man-portable gun and use it to fire a projectile that would readily penetrate all but the heaviest armor. The only comparable weapons were crossbows and longbows, both of which required great strength to operate, and in the case of longbows, years of training as well in order to reliably hit targets. The real advancement of the gun was not its lethality (all three could kill a man in one shot, right through chain mail, at similar ranges), but the fact that killing people with a gun was as simple as following a couple instructions carefully. A peasant with six days of training with a rifle was as deadly as an archer with six years of training with the longbow, and not to put too fine a point on it, was much easier to replace when he got killed on the battlefield. More importantly, the gun was the first true equalizer, a weapon that could be as deadly in the hands of a 5-year-old child as in those of a trained soldier in peak physical condition. Without the gun, the break with England that was the American Revolution would never have been possible; a team of trained archers would have put down any such peasant uprising easily.
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