3 sannins vs Itachi - 3 sannins win hands down.
Points to remember...
1. Most importantly, we haven't seen Tsunade fight all out yet. Hopefully, we shall see her true abilities soon. After all, we thought Jiraiya was Kakashi- level before we saw him fight Pain.
2. I believe Itachi lied to Kisame, saying that Jiraiya could kill both of them, as he was always a double agent, preventing Madara's attack against Konoha. But who knows? Cause if he was a double agent all this time, why did he try to kill Kurenai and Kakashi?
3. Yeah Itachi cut off Orochimaru's hand, but against 3 sannins at the same time, he wouldn't get that chance.
At the start of the fight, Itachi would become overwhelmed.
Katon jutsus and shadow clone explosions vs. (Jiraiya-Toad mouth traps, toads spitting fire, needle barrage, rasengan) + (Tsunade- earth shockwaves, raw taijutsu, offensive medical ninjutsu like the limb disorientation thingy used on Kabuto) + (Oro's - a million snakes, kusanagi sword, many ninjutsu)
So Itachi will have to turn to Susanoo pretty quickly, which takes up so much chakra, especially when fighting against 3 sannins.
So Susanoo + sword of totsuka + yata's mirror versus Katsuyu, Gamabunta, Manda (or other huge snake(s)). This fight will be so long and drawn out. Yeah Susanoo could defend against anything, but it is also attacking 3 huge, experienced summons, with 3 sannins who would have more chakra to fight at this point.
Amaterasu - well Orochimaru could use Rashoumon, or 2 or 3 of them. And if he get's hit, he has that last gasp special substitution that will drain all his chakra. I believe Tsunade can regenerate from anything. Jiraiya could die from Amaterasu, but he could also seal it away while the others fight, so it depends.
Meanwhile, MS genjutsu could take out any of the 3 in an instant.
So my conclusion is... 2 Sannin win (Orochimaru and Tsunade). Jiraiya dies from MS exhaustion cause Itachi would most likely target him. Itachi dies using Susanoo from exhaustion.
Cheers