People sometimes throw Sannin around as if it is a rank like Kage or Jounin.

Sannin translated to English means Three Ninjas. It refers to the three legendary ninjas, Jiraya, Tsuande and Orochimaru who all seem to be at or above the Kage level.

So no one can really be a sannin, except for those three.

As for Sennin, I think it means hermit, but it also can mean "master" or "sage." That can be a rank, but we've only seen Jiyara call himself Sennin.