which constitutional principle served as inspiration for the creation of the three branches in the U. S. federal government

It's the principle of separation of powers. This is an idea from the French thinker, philosopher of the Enlightenment Charles-Louis de Secondat, baron de la Brede et de Montesquieu (1689-1755), better known simply as Montesquieu. He wrote a book called The Spirit of Laws, a major contribution to political science.