In our three branch government, explain what is the name of the system that the Founding Fathers created to insure that no one branch would gain too much power?

The answer is Checks and Balances.

Checks and Balances is a political principle which describes how the branches of government work with each other. The Executive, Legislative, and Judicial branches each have a role in government decisions and events.

For example:
The Judicial Branch can decide if presidential acts are constitutional. This illustrates the Judicial Branch balancing the power of the executive branch by checking on presidential decisions.
The Legislative Branch can impeach the president, override a veto, and confirm executive appointments.
These are example of the Executive Branch being regulated by the two other branches of government.