Itachi unrestricted will be a tough fight for even both Sasori and Deidra, however the stipulation that he is in character gives the two former partners the ability to exploit Itachi's passive behavior in battle.
It's tough to say what exactly "manga knowledge" would mean for either side, being that members of Akatsuki have some knowledge of other members but perhaps not necessarily complete knowledge.
Itachi's knowledge on Sasori and Deidra
Sasori's poison is well known even by the Sunagakure. Itachi would not allow himself to be hit or scratched by any of Sasori's weapons. Itachi would also likely know about the Puppet Army, on the account of Sasori's self-proclaimed accomplishments with it. Itachi would likely know that Sasori himself is a puppet, or deduce it after activating Sharingan.
What he wouldn't know, at least not for sure, is about the heart container. Sharingan can perceive chakra, but its debatable as to what degree it can see the finer details. Most visuals of the Sharingan viewing chakra seems to depicting it seeing chakra in the form of clouds of energy. Sasori's chakra likely runs through his entire body, or at least through his general chest cavity
. It's questionable whether or not Itachi could discern that Sasori's heart container is, in fact, housing his heart. Sasori's Kazekage puppet and specific attacks such as the poison gas and iron techniques are also likely unknown to some degree.
The important thing to consider about Deidra is many of his techniques are specifically focused on fighting Sharingan Illusionary techniques
, especially Itachi's. There's nothing indicating that Itachi would know about this or about C0. Whether he knew about C4 Garuda or not is questionable, because even though Tobi seemed to know about it, he developed it specifically for fighting Itachi, so he may have kept it secret from all but his partner.
Deidra and Sasori's knowledge on Itachi
Because Zetsu knew about Ama, Tsuku
and even a bit about Susano'o
, it may be debated that all of Akatsuki knows of them. Nagato seemed to know
some degree about them as well.
Needless to say, they will be aware of Amaterasu and Tsukuyomi. They will also know about his general-Uchiha related abilities including standard Sharingan, genjutsu and Katon usage. Konoha and other village higher-ups knew about Susano'o as well, it's likely they will know about Susano'o, but perhaps not its specific attacks.
Itachi is typically passive in battle, typically letting his opponents make the first the attack. The only exceptions are when he's toying with his opponents or when he's forced to it through mind control. Deidra and Sasori, however, are unlikely to be effected by such genjutsu. Sasori, who has no brain stem or typical visual medium, is likely resistant to genjutsu, and Deidra will obviously start with his anti-genjutsu eye ready.
Sasori will not underestimate Itachi like he did Sakura and Chiyo, and will either start out with the Kazekage or Puppet Army. Either way, Sasori will likely keep his distance to avoid CQC with a Elite Uchiha with Sharingan prediction. Deidra's best move is to take flight, which is a likely scenario considering it's his typical fighting strategy
. Because of the constitution of Sasori's puppets and Deidra's clay bombs, combined with the previously mentioned tactics Deidra and Sasori will start of with, it is unlikely that genjustu, possibly including Tsukuyomi, will be Itachi's course of action.
Itachi will instead rely on Katon's, taijutsu and Amaterasu to battle his opponents. Itachi's general Katon's may not reach Deidra because of their range and speed, but Amaterasu certainly will. The problem being that Itachi must look away from Sasori to hit Deidra with it, and depeding on the angle of elevation, Amaterasu may end up hitting the clay bird rather than Deidra himself. That being said, Deidra's clay takes time to properly use, so Itachi will likely attack Sasori first.
Itachi's obvious plan would be to blast down Sasori and his puppets with ninjutsu. Katons would be effective at both destroying puppets as well as shielding Itachi from poison needles and gas because of the resulting explosions
. Itachi's Katons are more destructive than mere explosive tags, and more practical offensive-wise.
However, assuming Sasori has knowledge of Amaterasu, he will likely use the Kazekage as his initial puppet instead of the Puppet Army, as it is best suited for defending against Itachi's known ninjutsu's while keeping his distance. The Kazekage is also equipped with formidable melee weapons
as well as poison dispersion
. The Kazekage puppet's speed was also noted by a battle-experienced veteran.
Sasori will easily buy enough time for Deidra to prepare his clay weapons. Deidra is likely to jump straight to his anti-Itachi measures(C4 Garuda), due to his hate of Itachi
Itachi will not let Deidra so easily complete his giant explosive doll. The best solution is to risk taking his focus away from Sasori and try to hit Deidra or Garuda with Amaterasu while the bomb is being created. This leaves Itachi open, so he'll likely resort to KB feints, attempt a strong genjutsu in hopes of it working or go right into Susano'o as a precaution against both Sasori and the Garuda.
The amount of flames required to completely destroy Garuda would require a considerable amount of time. Amaterasu is the most surefire option, but the amount of MS-time required would leave him exhausted and give Sasori an opportunity he could not afford.
This leaves Susano'o as the best choice considering the amount of time needed. It provides him protection against generic explosions and weapon based attacks. However, the microscopic destruction Garuda offers, as well as Sasori's poison gas, would surpass Susano'o due to Itachi's inherit need to breathe.
Susano'o is also Itachi's best weapon, as the Yasaka Magatama offers him the range to deal with Deidra and the destructive power to overcome Sasori. The Sword of Totsuka is arguably not useful, because of Itachi's immediate need to slay Garuda. The Sword's extension length, at most, is only a few meters, and arguable shorter than this distance
. It may cut Garuda, but the clay can reform.
Itachi is left with launching Magatama's, a strategy that would potentially eliminate Deidra if he could not dodge and/or keep Sasori on the defensive. However, I sincerely doubt he can spam such a powerful technique, and I contend that in around 3 uses he would be greatly depleted. The question is would that be enough to stop both Sasori and Deidra, while leaving Itachi enough stamina to fight the remaining contenders.
Overall I give this match to Sasori and Deidra with Med-High Difficulty
. I feel that their immunity to genjutsu and their air-born attacks are a perfect offense against Itachi. Sasori's defensive capabilities and Deidra's long distance fighting make them a powerful team, and Itachi's long range offenses, while powerful, will be too taxing to take out both of his opponents and leave him in a position where he won't be too exhausted or open for counter attacks.