Then why was Black Zetsu lecturing White Zetsu on what Amaterasu, Tsukiyomi and Susano'o were. and why was White Zetsu explaining Kirin to Black Zetsu?
@bold: Both are figure of speeches that convey the same meaning.
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Notes he says "Once BOTH Tsuki and Ama". This alone shows Susano' requires them, add this to the Databook entry that Kishi himself wrote, you have more than enough confirmation.
The only time it was said you only need two powers was by Sasuke, and this was shorty after gaining MS. He knew nothing about it at the time because he hadn't read the stone tablet, so his word isn't credible. Accept defeat now?