That's a good question. When it comes to invention, I make a distinction between things that exist in nature and have been harnessed by man and things that don't. If I had to pick three off the top of my head I would say the wheel, written language and government.
My third choice was a difficult one and, probably, controversial as it is also one of mankind's worst inventions. However, government is the foundation of civilization. How much of what man has accomplished would have been impossible without it?