Putting the whole scenario into perspective, a straight out fair fight between the sannin and the uchiha brothers would probably end in a stalemate although I am leaning towards the Uchiha brothers.
A quick analysis of everyone's abilities:
Sasuke - Kirin (In a fight like this, almost useless as he would need to make his own clouds, thus using up more chakra than needed, in his right versus Itachi, Sasuke used dragon flame bomb 6-7 times before a decent majority of clouds gathered.)
Amaterasu (Assuming both Uchiha will have the Eternal Mangekyo Sharingan would mean that Sasuke would overuse his blaze release rather easily, thus wasting away even more chakra (despite the fact he has almost none to begin with.))
Sasuke may or may not have Tsukuyomi, since it has never been comfirmed, therefore for the purpose of this fight, I will assume that he doesn't have it.
Kenjutsu (We can say, without a doubt, Sasuke's swordsmanship skills are extremely refined, he was able to at first keep up with 'Master Swordsman' Killer Bee, and also annihilate Suigetsu (although he is far from Sasuke's level to begin with). In this fight; however, I doubt a sword would be too useful.)
Lightning and Fire releases (Sasuke's elemental types are fire and lightning, his entire fighting style is based off overusing his sharingan and spamming fire and lightning type attacks, whilst Sasuke may be smart, he tends to waste most of his chakra as seen in his fight against Itachi, Deidara, Orochimaru and to an extent; Killer Bee (pre-transformation). Also, Jiraiya is a fire type user so we can assume that he would be able to see Sasuke's fire attacks coming, the chidori can be cancelled by a simple rasengan and his chidori blade could easily be avoided as long as the sannin keep their ground.)
Itachi - Tsukuyomi (Itachi probably wouldn't resort to using Tsukuyomi in the middle of the fight, if he uses it on any of the 3 sannin, he may be able to incapacitate them rather easily (assuming he's even able to have direct eye contact.) but this would also temporarily incapacitate Itachi, which leaves Sasuke fighting two sannin. This would probably lose them the fight.)
Amaterasu (Same as Sasuke's although can't be manipulated with ease, also used very, very sparingly.)
Susanoo (Probably Itachi's biggest asset is the Susanoo, with one hit, he easily sealed away Orichimaru, and a shield which reflects all attacks is a nice additional bonus. Assuming his Susanoo is similar to Sasuke's, Itachi's dexterity and agility would be lost, meaning that the titan like Tsunade or Sage Mode Jiraiya may be able to scratch the Susanoo (Since a kirin was able to do so.)) Assuming that Itachi can use it for extended periods of time, it's a losing battle for the sannin, but with three huge summons, three great ninjas, an army of undead and genius battle tactics gained from experience, I'd say Itachi would reach him limit soon enough.)
Intelligence (Itachi, one could say, is as smart as Minato, having extremely complex plans, which have more plans tied into it, is a huge conspiricial plot. He is able to plan things far into the future, personally I would compare Itachi's xenatos gambit to that of Lelouch from Code Geass.)
Sannin -
Summons (Whether it be frogs, slugs, snakes or undead, Itachi and Sasuke would be swarming with thousands of potential enemies, I personally can see Itachi and Sasuke putting up a fight although Itachi and Sasuke would be worn out rather quickly, I say that the sannin has much more chakra than Itachi and Sasuke, meaning that if the summons don't give them a chance for an attack, nothing will.)
Strength (Jiraiya has Sage Mode, Tsunade (while seemingly useless.) is still hokage and also has monstrous strength. Orochimaru is immortal, his substitution can save him from amaterasu and he was playing with a out of control 4 tailed Naruto...)
Teamwork (The sannin are the sannin because they work together and compliment eachothers abilities, the sannin are very strong individually, but together are a force to be reckoned with. Itachi didn't want to fight Jiraiya (although this was probably due to him not actually wanting to capture Naruto.) They were able to fight on par with Hanzo a few decades ago, so with an extra decade or two of experience, they are worthy of defeating Itachi and Sasuke.)
Due the that quick synopsis, I would assume that the fight would easily end in a stalemate, although both sides could win, I would say the Uchiha brother's have a slight advantage although I assume it really depends on the battleground and what plans Orochimaru and Itachi make up.
Yiin
