1) how does  the legislative branch check executive and judicial branch? 2) how does the executive branch check the power of the legislative branch and judicial branch? 3) how does the judicial branch check the legislative branch and the executive branch?

1. The legislative branch checks the executive branch by being the one who can call for impeachment (the process starts in the House and then the Senate approves if the reason for impeachment is reasonable) and vetoing laws the president approves. The legislative branch also checks the judicial branch by approving appointed judges by the President and can propose constitutional amendments to overturn decisions made by the court. 
2. The executive branch checks the power of the legislative branch by also vetoing laws the legislative wants to pass. It also checks the power of the judicial branch by being the one who appoints the judges and they can technically choose whether they want to enforce the rulings and decisions the court has made.
3. The judicial branch has power both over the Legislative and Executive branch by declaring if one of their actions is unconstitutional and they have the last say (judicial review).

He advocated for a separation of powers of the government and the ones enforcing the law.