The Sannins refer to the team trained under the 3rd Hokage (but back then he wasn't a hokage yet). They are Jiraiya, Tsunade, and Orochimaru. And they are all suppose to be 50 years old.

They are Legendary because they all are very very very powerful. i guess not every memeber of a team is going to be strong (e.g. sakura is a weakest in team 7). each sannin is near impossible to defeat cuz that's just how powerful they are.