Halo 3 backwards is about waking from cryo sleep, jumping a exploding spaceship out of a jump node, joining the two parts of the broken ship together automatically, having some guy undeclare you as a hero, bringing dead evil aliens back to life, unallying an alien friend, and some moar random crap.

Halo 2 backwards is about you resurrecting some dead evil aliens, and ????? and having some officer snatch away your medals and stuffs.

Halo 1 backwards is about unexploding a spaceship, undiscovering the Flood, repairing enemy vehicles, forgetting an epic secret, and going back to cryo sleep.