If you can remember the Sasuke vs Itachi battle, when Itachi came in to poke Sasuke's forehead Itachi was nearly or completely blind. But my point is at this moment he DIDNT have his Sharingan activated.
It seems like once you unlock Susanoo you don't need to use Sharingan to use it again, since it's an special chakra and not an eye jutsu on its own.
Even Madara used Susanoo while in Rinnegan.
And Itachi breaking in the cave with Susanoo having 3-Tomoe
Sasuke only needs EMS for his combined attacks of Amaterasu.
Itachi still had his sharingan active, it was just faded, his Susanoo didn't go out until he died. His eyes are shown to be white, brighter than When Sasuke was going blind so it was less blind. Itachi-http://narutobase.net/manga/Naruto/393/4 and Sasuke-http://narutobase.net/manga/Naruto/484/10 and
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Sasuke was over using his MS more than Itachi is why he was going blind so fast, his eyes are darker than Itachi's and his Susanoo went out as he reached his Sharingan's limit. Both Sasuke and Itachi could still see as shown by how they reacted to attacks from their opponents. Susanoo are MS jutsus, Sasuke said only those who control the MS can use Susanoo
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As for the Itachi using 3 tomoes, That is a possibility but it doesn't show his eyes when he was using Susanoo's arm, and when it did show his eyes the chakra behind him doesn't look the same color so it could have been dissolving as he went from MS to 3 tomoes.
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and
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. He used his MS to use Susanoo when Sasuke was chasing him
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