The MTV publicist has told us that the finale (with winner reveal) will be Tuesday, November 21, before the premiere of Season 31 -- yes, they are back to back on the same night -- but then her predecessor had told us that the finale would be November 7, so we know they do lie.
I believe the third part is gonna be a behind the scenes of the two-part reunion. Streeeeeeetcccchhhhiiingggggg!