Why did the Virginia delegates insist that the new government have three branches?

The Framers believed that powers should be shared among three branches because they didn't want one branch to become too powerful, resulting in complete tyranny of the masses. They were especially wary of the national executive, or President, as they had just declared their independence from a powerful king and wanted to make sure that the United States would never have to deal with such a problem again. For this reason, they intended for Congress to have the majority of the power in the government, yet the President has become more powerful over time.