they cant become sannin that title was given to them by a dude they faught dont remember his name.
I guess they would need a powerful influential ninja to call them that like Hanzo was, but otherwise it is plausible providing Sasuke returns to the village, although that part isn't that plausible at all.
Sasuke = Orochimaru
Naruto = Jiriyha
Sakura = Tsunade
they already surpassed the sannin except for sakura