Ans this is the way I understand it
All double MS (or EMS) users has susano'o, tsukuyomi, and amaterasu (after training of course)
also, each eye has a unique ability of it's own
Obito's (Kakashi) eye- Kamui
Shisui's eye - KA
and as for Madara, Itachi, and Sasuke, the unique ability is an extent version or an expertise of each one of the three techs (susano'o, tsukuyomi, and amaterasu)
Sasuke- Amaterasu (enton)
Madara- Susano'o (PS)
Itachi- Tsukuyomi (we all know itachi is the king of genjutsu and he had the strongest tsukuyomi by now, right?)
