I don't see what keeping 3 QBs accomplishes either. If Dak isn't available none of the backups are saving the season. Who the backup is, isn't a clear cut decision.
Rush- I can't just forget the performance he had against the Vikings last year. Technically he has the most impressive in game resume of the group. But he hasn't had a very good offseason.
Grier- A former 3rd round pick that may also be the most naturally gifted of the group. Maybe unfair, but the QB needy Panthers had no interest in keeping him and his 2 NFL starts were terrible.
DiNucci- The guy does play with confidence and is a true gunslinger for better or worse. Looks better this year, but needs more work.
DiNucci is probably the most likely practice squad candidate. Rush, could end up somewhere else namely the Vikings that have an uninspiring backup QB battle themselves.