With all of these three political thinkers, a major theme was the idea of the social contract. All three felt that the only way that a government could be legitimate was for it to be based on the consent of the people. They believed that the people consented to be ruled by the government in return for the government protecting their rights to (in Locke's words) life, liberty, and property.
The Constitution was written in such a way as to reflect this contract. The government is prohibited from taking away people's life, liberty, or property without due process of law. The government is also set up in such a way (separation of powers, which was Montesquieu's idea, etc) as to make it less likely that it will try to take rights away.
Thus, the Constitution is written to reflect the idea of a social contract between the people and their government.