Itachi could just use susanoo/Yata Mirror to protect himself from minato speed since Tsukuyomi and Amaterasu wont work on him so Itachi will be protected .and he can just take out Nagato with Tsukuyomi or his Sword of Totsuka since Nagato mobility is very slow then he can surround him self with Amaterasu flame just as Sasuke( its most likely that he has this ability due to Sasuke having the same abilitys as Itachi).
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Itachi can't defeat both nagato and minato at the same time.