He was going to play a 4th character as well because that was part of the conditions set by Colombia Pictures otherwise they would refuse to fund the movie, as he was also supposed to be Maj. "King" Kong, who was eventually played by Slim Pickens

> It’s well known that Peter Sellers played three roles in the film: Group Captain Lionel Mandrake, President Merkin Muffley, and the titular character, Dr. Strangelove. But Colombia Pictures originally set a condition for Kubrick: The studio would only finance the film if Sellers played at least four roles in the movie.

> In addition to the three aforementioned characters, Sellers was originally meant to play the Air Force Major T. J. “King” Kong, but Sellers was reluctant since the beginning. He initially voiced his concerns to Kubrick stating that he was worried about the workload and was afraid that he would not do the character’s strong Texan accent justice. Kubrick managed to convince Sellers to take on the role and even elected co-writer Terry Southern who was raised in Texas to help Sellers with his lines. But once filming started, Peter Sellers fell on set, exacerbating an injury the night before and was unable to continue with filming in the cramped cockpit set.

https://www.metaflix.com/how-peter-sellers-lost-his-fourth-role-in-dr-strangelove/