it's Ghidrah, the Three Headed Monster (1965)! one of my favorite Godzilla movies, and the first one I ever "owned" on 8mm film.
actually, it was just the first Godzilla vs Rodan vs Mothra bit, but it was still pretty cool. this one has all the great stuff you want, monsters, spies, Martians, gun battles, assassins, loads of humor and some great effects. it's awesome!