the reasons as to why the Sannin would win this fight is because
1. the sannin are very experience
2. they all have giant summonings and more and Itachi has nothing really ass huge, he could get his susanno which would be broken from tsunade's strength or the summonings
3.Itachi is strong, but not that strong to handle all three lengendary sannin Jiraiya took pain on by himself and took out three which Itachi could probally not do, and could of killed him if he knew there were more than the ones he fought, Orchimaru is supposly the strongest of all three and if he is he stonger than Jiraiya and like I said Jiraiya himself is extremly strong,and Tsunade she has this monster strength that would kill Itachi in an instance, her incredible healing powers that heal any wound even ones that seem impossible plus more which we don't know yet because they underestimate her because she doesn't really do anything, but she isn't one the Sannin for nothing and like they say the quitest is alway the best I mean who knows she probally has more summonings than Jiraiya and Orochimaru put together because she summons slugs and im pretty sure there are more slugs than frogs and snakes and im positive Katsuyu isn't her only summoning even more maybe sage mode and that goes with my reason back why Itachi can't beat the Sannin because him and his brother can barely beat kabuto in sagemode and Tsunade is stonger than Kabuto so in her possible sage mode he has no chance and she probally can summon snails which the hard back cousins of the slugs.
Therefore Itachi would loss and if Sasuke was there too he would lose too.