1- Really going to fight him? So what, he was only toying last time he got pwned in 3 seconds and wanted his body? Puh-lease! And what you fail to realize is that Jiraiya has to first GET into Sage Mode which, as we now, still takes some time... Time that he may not get because in case you're forgetting... Itachi knows virtually everything about these 3 since he was the ANBU Captain of the very same village they come from. And even Orochimaru himself stated that the Sharingan is his weakness... So no, he wouldn't even come close to giving Itachi a run for his money. Even in his strongest form when Itachi was at his weakest point, he got pwned and sealed away forever. Itachi >>> Orochimaru. Deal with it.
-Itachi is stronger than Orochimaru i never said Orochimaru was stronger, i said he would give him a run for his money. Orochimaru wouldn't fall easy to Itachi after what happend the first time, plus he knows about his genjutsu, and how will Itachi counter Hydro mode, Manda, and Orochimaru and his two Edo Tensei (Kage level shinobi).
-Also Jiraiya easily entered Sage Mode, after protecting himself from 4 different summons of Pein, and one of his bodies, and he still quite easily avoided any damage, and entered sage mode. So don't come and say he wont even get into sage mode you idiot.
2- As I stated previously, Jiraiya has to first GET into Sage Mode whilst it'll basically be 2v2 since Orochimaru has already shown he'll get pwned within 5 seconds like their previous encounter... And he doesn't even have to make eye contact with Itachi since the man can cast Genjutsu at the point of a finger. As for the Rasengan, I hope you realize the speeds at which Jiraiya will be moving, as well as the speed of Kisame who doesn't even have to get close to Jiraiya because he'll just drown if he stays in the dome for too long.
-Wow stupid, Jiraiya as stated before by me, would easily enter sage mode and still be able to aid his friends with Gamaken and Gamabunta, which would pretty much own the freakin Water Dome. Also Jiraiya in SM can use Goemon which would reduce anything to cinders which also means it would make the water dissappear because of the immense heat from the boiling oil. As for Genjutsu, all Itachi's genjutsu is eye contact you idiot, also Orochimaru wouldn't be that stupid to do the same thing again and Orochimaru is hell smart and he would seriously kick butt. Kisame is out of the question he would fall easily to Jiraiya so it's 3 vs 1.
3- So Tsunade's healing factor basically means she'll be Godly for a few minutes in a 2v1 fight since Itachi can pwn both Jiraiya and Orochimaru in about 10 seconds flat... And we all know her healing factor won't do much against Samehada which'll just absorb her chakra faster or things such as Tsukuyomi, Amaterasu or Susano'o which can seal her away.
-Kisame would be out by Jiraiya easily, it is already stated Jiraiya would have pwned Kisame. Also Jiraiya and Orochimaru together would wreck Itachi, seriously is there no one here reasonable to see that the two legendary sannin will outmatch Itachi. Samehada will be of no use because of natural chakra. Tsukoyomi would fail since they all know of this after Itachi revealed himself. Also Amaterasu would be a problem but Jiraiya can seal it and and jiraiya knows of it so he would have told his teammates. Susano = stay away from Itachi until he is out of chakra.
Itachi has almost no freakin chakra compared to the sannin, he would be able to use like 2-3 MS moves.
So no, my answers do work. The 3 Sannin die with Itachi being almost out of Chakra, but he can simply get some from Kisame and/or Samehada because he'll still have heaps of Chakra left in his reservoir.