The way the government is set up allows for checks and balances. We have the judicial (Supreme Court, who decides whether or not laws are constitutional), legislative (Congress, our local representatives), and executive (President) branches. While it may seem complicated, this set-up creates a system wherein no one person can gain too much power, and it was set up as such to avoid problems that people originally had by monarch rule.
If we had only one government part, then that government might turn into a dictatorship since there would be no other part to balance it. If we had only two parts of government, then the two might disagree with each other, and there would be no middleman to settle the dispute. If we had four or more parts of government, the system may become too diluted and complicated, and due to the confusion, nothing would get done. With a three part government, there is a system of checks and balances. No one part holds too much power, and if two parts disagree, the third can act as judge. Thus, we should have a three part government.