That's a very strange question... there must be hundreds and hundreds of boxers, who fit this description!
Andy Price had wins over Pipino Cuevas and Carlos Palomino before they were champs. Willie “The Worm” Monroe and Bobby “Boogaloo” Watts both beat pre-champ Marvin Hagler.
Leo Houck beat 6 future world champions: Jack Britton Frank Klaus Battling Levinsky George Chip Jack Dillon Johnny Wilson
Tommy Morrison beating Big George Foreman immediately sprung to my mind. (I don't consider WBX or IBX or WTF belt holders as Champions).
Kenny Lane - beat Carlos Ortiz, Carlos Hernandez and Ralph Dupas Alfredo Urbina - beat Jose Napoles, Eddie Perkins and Chango Carmona It's not easy when you draw the line at 3 like I did. It gets easier if you go less.
Elmer Ray: Ezzard Charles/Jersey Joe Walcott Lloyd Marshall: Ken Overlin/Ezzard Charles/Jake LaMotta/Joey Maxim/Freddie Mills Davey Abad: Tony Canzoneri/Henry Armstrong
Kid Tunero - Ezzard Charles, Ken Overlin, Marcel Thil and Anton Christoforidis Spider Webb - Dick Tiger, Joey Giardello and Terry Downes