Kabuto a student of tsunade ?? where d heck did u read that??
Tsunade did not have any student except maybe her constant companion a girl. Nowhere in manga was it mentioned that Kabuto was a student of tsunade. Tsunade was long gone from the leaf village before Kabuto ever arrived there. Kabuto is a minion of orochimaru.
The 3 legendaries are Orochimaru - snake , Jiraiya-frog, and Tsunade -slug .