sannin = three ninjas

they're called the legendary sannin cuz they are all very skilled and talented ninjas.

who cares who gave them the title, it's just given.

it's not that only 3rd and sannin are allowed to mess with kyubi seal, it's just that only they are knowledgeable and skilled enough to handle such thing. but i'm sure some others might be able.

and try searching for your answer before posting questions that have already been answered.

EDIT: heh, i guess someone else got to it first.