Great point. I'll bet everything I've got it has never happened before, at least in the era of the forward pass.
Feel pretty confident in this b/c from 1991-2011 the third QB rule meant that #3 QBs weren't in games at all unless the first two went down, and that rule was put in place because before that people weren't dressing their third QBs because they practically never used them and it was a waste of an active spot.
Basically, if we limit ourselves to the era of the forward pass we can almost automatically exclude everything before 2011.
So post 2011 we'd need to find: 1) a #3 QB who was the primary kick returner, 2) a #2 QB who played the first offensive play but not the second, and 3) a #1 QB who then entered the game.
I can't think of #1, and the only scenarios for #2 are a #1 QB who is injured enough to be the backup but not to start and the #2 gets injured on the first play of the game or the high school silliness Rivera engaged in yesterday.
Maybe there's another #3 QB/KR I can't think of, and maybe that injury scenario and high school silliness happened too, but both of them together, no friggin way.