I don't know anything about the third turbo but it won't be a simple supercharger, rather an electric supercharger.
SCOTT confirmed it wil be an I6, it is not 100% sure. When SCOTT said the 1M E82 was getting a new TT I6 hat would be further developped for the M3 F3X, at the end it was nothing else than a Z4 sDrive3is engine, that indeed is an N54 slightly moddified by M GmbH. It is not SCOTT how decides whether it is going to be a V6 or I6. I yet keep my hopes for V6.