They have to be one of the few groups to have hit songs with at least four different vocalists. Don and Glenn, of course, but Tim on “I Can’t Tell You Why” and Randy on “Take It to the Limit.” Joe, Don and Bernie all took lead duties on some songs, but I don’t think they were ever hits (of course, Joe had his own hits).
Chicago, at their peak had three (Cetera, Kath and Lamm). Hard to think of bands who had more than two vocalists.
The Beatles, didn’t Ringo sing Yellow Submarine?