Schwarzenegger and DeVito played fraternal twins Julius and Vincent Benedict in Twins, which opened in theaters in 1988. Triplets will see the brothers discover they have a third sibling, portrayed by Morgan.
Schwarzenegger had nothing but praise for Morgan on Jimmy Kimmel Live!
"Tracy is a fantastic guy," he said. "I met him a few times, and then we got together with this. I mean, he is a fantastic entertainer, great actor, great energy, and he is also the age where we can mainly make him a little older to sell the idea that he's the same age as Danny and me."
Schwarzenegger also said Triplets has a "fantastic" story that was years in the making.
"We've been tinkering around with that story now for 10 years, and then when [original director] Ivan Reitman took charge of it again ... he really got into it and he straightened it out," he said. "We have a straight 10 script. We love the script."
Schwarzenegger confirmed that he and DeVito often prank each other set and recalled a time when DeVito tricked him into smoking marijuana on the set of their 1994 movie Junior. Schwarzenegger said DeVito cooked Italian food during a lunch break and then offered him a cigar.
"I'm smoking it and it was fantastic. We get to the set, we continue on with the scene. I forgot my lines," the star shared.
"Find out then an hour and a half later when I finally started remembering things again that Danny put some marijuana in the front of the cigar," he said. "I was stoned out of my mind."
Triplets will begin filming in the fall.