sannin...wtf

Okay, seriously, Sannin is not a rank. Just a title. There are no special powers to being a sannin. They are just the three ninja who where trained by the hokage. Nothing more.

but they are the only ones who can leave the village at will.

Actually, no. That's only after they've retired from ninja life. Orochimaru left while he was a ninja, and he became a rogue. Jiraiya and Tsunade left once they were done with their ninja lives.

Sennin = Sage
Sannin = Short for Densetsu no Sannin; Literally meaning "Legendary Three Ninja".

This might help you

... Actually Sannin, is a rank. It wasn't given by the Hokage at all. In the manga it says that when they where fighting in Ame, during the war, they where given the title by the leader of Ame for the fighting skills, not because they where trained by the Hokage...

Really? So it's just a coincidence that the three sannin (sannin means three ninja) were the three who were trained by the hokage?

Yep. Total coincidence, otherwise Hizuren (that's the Third Hokage) and those two councilor people would also be called Sannin since they were trainedby the First and Second, as would Kakashi, Obito, and Rin, because they were all trained by the Fourth. So yeah, Sannin is a rank refined only to Jiraiya, Orochimaru, and Tsunade, not to any ninja trained by the Hokage.

Well okay, I guess you learn something new every day ^.^

Sannin is a title not a Rank.

There are only 6 Ranks: E, D, C, B, A and S

Sannin was given as a honourary title in Cannon and is not a position. On US they are considered high level S Rank chars comparable to Anbu or Akatsuki that have proved their skill and dedication to the site

Jiro put it perfectly...

Archived.