The American government was created with a three branched structure so as to make distinct the separation of powers under the Constitution. This in turn makes it easy to for to check each others power through checks and balances and avoid any abuse of power. Each branch has its own purpose and no branch has all the power. Visit http://en.wikibooks.org/wiki/United_States_Government/The_Three_Branches, for more details.
The government was created with three branches to correct its previous ways. It was originally set up mirroring British Articles of Confederation. When found to be too weak, was changed to three branch system of legislation, executive, and judicial for checks and balances among other changes.