The Three Wise Men are three spiritual entities that live in Finn's gut. They appear at the end of the episode "Ocean of Fear," where they help confront the Fear Feaster by telling Finn that every hero has a flaw, and that he is the greatest hero in Ooo.
There are 3 of them, they all are blob like spiritual beings that can float in the air. The one on the right has blue hair and a blue beard with a viking hat on top. The one in the middle had a long mustache and large nose who also wears a crown on his head. The one on the left has a think mustache with a very long and odd eyebrow, he wears a beray on his head.