They may be forced to. If Hundley is maybe a week or two, or three away from being ready to play at the end of TC they will have to keep someone to be #2 those weeks.
I agree that if Hundley is completely healthy there isn't much reason to keep three, since their third this year isn't like Hundley was last year, a guy you didn't want to lose. They wouldn't have three active on game days, so their 3rd qb can just as well be on the practice squad.