Willis and Schwarzenegger share a scene with the Rocky actor in the upcoming action film The Expendables.
Speaking to Entertainment Weekly, Stallone said that the three icons go "nose-to-nose" in the movie.
"Bruce and Arnold just got it. It's so funny because you get there and it's like, we've all seen each other socially, but this is different. It's like meeting a fellow athlete in the ring," the 63-year-old explained.
"Oh, OK, everyone knows how to work the camera and what angles work for them. It was just fantastic. They're such pros. Because it's probably never going to happen again."
The Expendables, also co-starring Jason Statham, Jet Li, Dolph Lundgren, mixed martial arts champion Randy Couture and Terry Crews, opens August 13 in US cinemas.