“Now a mediator is not a mediator of one, but God is one.”
King James Version (KJV)
3:20 Now the mediator is not a mediator of one - There must be two parties, or there can be no mediator between them; but God who made the free promise to Abraham is only one of the parties. The other, Abraham, was not present at the time of Moses. Therefore in the promise Moses had nothing to do. The law, wherein he was concerned, was a transaction of quite another nature.
Ga 3:20 Now a mediator is not [a mediator] of one. A mediator implies two parties between whom he acts. But God is one. The idea is that when God makes a promise by his sovereign power no mediator is required. God acts alone. Thus it was when the promises were made to Abraham. God, too, is One, the same, always, and hence the law is not due to a change of the divine mind.