I LOL'd , Gaara won't last.
They can do Shit to susanoos , Multiple Yasaka magatamas and Arrows will be Shot with high speed . , i Still can't figure out What is tsunade doing here.
Of course you will say they win because you Ignored Itachi's main fighting Style , Genjutsus (including Tsukyomi)
Itachi can Shoot amaterasu while Sasuke will control it., It will burn them.
either one of them can control katsyuya .
itachi's main fighting style as you said is genjutsus,which require eye contact,gaara took on 5 ems's at the same time without looking at their eyes,onoki did the same, and so did tsunade who still pummeled them ontop,so abt 15 sharingans(ems) they fought between themselves and they didnt look into their eyes at all,so not looking at one ms and one ems while fighting is a piece of cake for these three.gaara faced his susanoos without even using any sand sensing or third eye as far as we know,so those 2 techs combined with his skill of fighting sharingans already shown mean genjutsus is basically a moot point,not ignoring itachi's fighting style,itachi himself claimed all techs have a weaknes,his own is eye contact and gaara is the perfect counter.
Gaara has shown the ability to attack several shinobi at once
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All three kages were smaller and faster opponentsand more difficult to hit than susanoos,yet they all would have been hit if not for gold dust wall above.
Both thier susanoos have holes under,gaara has shown the ability to raise sand from under 3 kages at once and entrap them
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catching 2 opponents at once,especially since their susanoo reduces mobility is very easy for gaara. Itachi's yasaka is blocked easily,sasuke's yasaka is basically enton which wont harm the sand,a dual layered sand wall easily parries those two attacks. Susanoo arrows were hampered by a medium sized pillar,kimi's normal bones resisted and cut through chakra infused blades,his strongest bone was stopped by 13yr old gaara's sand,and it not only tried to pierce,but continously drill through for a time and then shattered like glass,those arrows are either blocked or diverted like danzo did,both gaara and onoki have shown reactions enough to counter madara's yasaka,so itachi and sasuke's are not a problem.itachi being healthy dosent give him kisame-level chakra,ms still hurts like hell,and his vision wll get blurry continously.
Onoki knows abt amaterasu through gaara,he will take flight instantly,which is what he does normally anyway,ama is instant from short ranges,not long,it's a short ranged tech,a flying onoki will have no problem,even if he stays on the ground,gaara will protect him for the few secs needed to form jinton and obliterate the susanoos. Onoki can easily use his rock clones to fight for him while he forms jinton.
Yata mirror is endowed with all basic nature transformations,and can turn it's nature into a property suiting the purpose,jinton is a kekkai tota,sand isnt a basic nature transformation and it's weakness oil isnt either,yata isnt going to reflect anything,it will just be a normal shield. Onoki can use his earth abilities to easily raise the ground above sasuke and itachi,thay will knock them off balance despite having susanoo as it has a hole underneath.he could easily make a gian rock golem appear right under their susanoos,they will be screwed by that time.this is also prime onoki,80 year old onoki could do many rock techs 2 jintons before running out,he laso matched a younger,edo muu's jinton,prime onoki would do at least 4 jintons and quite a lot of rock techs,chakra isnt a problem for onoki,and definately not gaara.
You wonder what tsunade is doing here?,i'd like to see sasuke and itachi actually have the breathing space to face her in this fight while having gaara,who his casual sand tsunami made his dad mistake him for shukaku,meaning his sand powers are bijuu level,and a prime onoki who is even more haxed than the current old one. She would simpy summon katsuya,slugs dont exactly have eyes that the uchiha bros can use to put her under genjutsu,and she can split into hundreds oc clones,each capable of tanking CT and CST,and taking the heat and corrosive chakra of naruto's 4tk,6tk and even 8tk easily. Tsunade can attack itach's susanoo from behind,yata mirror dosent block the back,she already sent 2 full susanoos to the ground with one blow,danzo when fighting sasuke noted the weak spot of susanoo which was at the back,tsunade's punches whether she knows the weak point or not will still hit there and break it,she moved faster than mei,a kage could form handseals,she isnt slow at all. Itachi will either have to turn around and face tsunade with bleeding eyes and blurry visin,and risk turning his back on either prime onoki or gaara,both will prove fatal.she can even stay out of the battle,summon katsuya,and have her give them chakra from her own to strengthen them.