The Sannin, translated to "Three Ninjas," are legendary shinobi taught in Konohagakure by the 3rd Hokage. They were given their title by Hanzo the Salamander, the renowned leader of Amegakure whom was impressed by their teamwork. While very powerful individually, their abilities would increase 3 fold when they fought together; it's said that the battlefield was their own personal stage.
However they were uniquely flawed as any human and it was their own personal demons that drove them apart. Tsunade had a gambling problem and developed hemophobia; Orochimaru was egomaniacal and psychotically ambitious; and Jiraiya was lecherous and suffered from depression.
Ironically, they'd each go onto mentor the members of Team 7: Jiraiya trained Naruto, Orochimaru trained Sasuke, and Tsunade trained Sakura.