Only numbers 50 through 79 are ineligible in college football. He can wear 90 in the backfield and not need to change numbers or report, so long as he is not lined up at an ineligible position (guard or center or some tackles) when the ball is snapped and no other offensive player on that play is wearing #90. The rules are similar but much different for the pros due to smaller rosters and the preference of TV knowing who is playing exactly what each play. This is one of the differences.
Gus had a ridiculous play that never made into a game situation thank God, where the QB3 would switch jerseys to high number in the eighties or nineties and line up at tackle uncovered on the field side with no WR on that side of the line. QB 3 would thus have to report as eligible but look like a tight end at first glance. Who really knows what a third string QB looks like anyway through a face mask? QB2 would line up in the slot in a diamond formation of WR on the boundary side. At the snap, QB1 would fumblerooski, which was picked up by QB3. QB2 would read the defense and either streaked as a WR or shallowed in the backfield based on safety rotation or keys. He would shallow if he saw a single safety, and QB 3 would zip a back pass to him. Meanwhile QB1 after the fumble would run field side on a streak. If uncovered, QB2 would hit him in the open. If covered, QB2 would read WR1 for a hitch or rush for whatever he could get. He ran it a couple of times in high school. Where it belongs.
We did use three QBs on a couple of plays in 2008 using similar creative substitutions designed to generate confusion in part hoping to exploit the opponents complacency, fatigue or lack of familiarity with the rules. Those plays were reverses with Bower and Whitmore both having an RPO after lining up as WR. It worked once. At UAB iirc, Clay or maybe Sears caught like a 40 yard pass. Sears was a former big boy QB who walked on as a TE, in part to be available for that stupid fumblerooski play I confusingly described above.
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