Mike Tyson -- I'm NOT in the New 'Hangover' Movie

Zach Galifianakis' face can rest easy -- Mike Tyson tells TMZ he won't be punching the actor's lights out in the third installment of "The Hangover" in Tijuana ... 'cause he ain't in it.

Mike was at LAX yesterday when he told us the disappointing news ... meaning he won't be following up his 2 memorable "Hangover" singing performances (he did "In the Air Tonight" in the original and "One Night in Bangkok" in the sequel).

But Tyson was more focused on paying tribute to his fallen colleague Hector "Macho" Camacho -- and praised him as one of the best fighters of all time.