Some time ago, I wrote about, and showed, the only known time that the 3 Howard Brothers appeared in a publicity photo.
Only 2 ever appeared at any one time in the troupe. Shemp was an original member, left for his own career, and returned when Curly became ill and had to leave. Shemp remained until his death in 1955. The Stooges still had 4 films left in their contract, so they hired someone to stand in for Shemp in those films. They mostly show this "fake Shemp" with his face covered.
Someone stitched together pieces from these 4 films, showing many (maybe all!) of the footage with Joe Palma, the poor guy who had to fill-in for Shemp Howard.
After these films, the Stooges got another contract for more short films, and made them. For whatever reason, they hired an actor named Joe Besser to be the third Stooge. Few people today know these films exist. They just assume that the Stooges always consisted of Larry, Moe and Curly. Shemp and Besser, and even "Curly" Joe Derita have pretty much been expunged from memory, which is quite sad, especially for poor Shemp, who deserved so much better.
Herewith: A clip from a Joe Besser Stooge film.