What are the three parts of the government of the US?

If you mean branches, they are the Legislative, Judicial and Executive Branches, which correspond to the Congress, Supreme Court and the President, respectively.

The Legislative Branch is collectively called Congress, which is a bicameral branch separated into the House and the Senate. They are responsible for proposing laws.

The head of the Judicial branch is the Supreme Court. Headed by 9 Supreme Justices, they are responsible for judging important cases, yet even more important, they are the leading authority on interpreting the Constitution and making sure passed laws are constitutional.

The final branch is headed by the President of the United States. He may veto laws, (though this can be overturned by majority vote), pardon criminals, and utilize a number of other executive powers under the supervision of the two other branches. He or she is also responsible for international affairs and maintaining the general welfare of the nation.

The system of checks and balances which exists among the three branches ensures, (at least in theory), that one branch does not become too powerful (a reference to times of anti-monarchy, when the documents were written). This system is not perfect, though works (in theory) well enough.