Derek Carr is a solid B, but the guys behind him are just not trustworthy options. None of them complement Carr play style so if they do get on the field the playbook is limited. This is the exact opposite of what you want from your backup.
It is not surprising that the Raiders are thin at quarterback as most teams still struggle to find a starter, let alone a backup. It is just curious that they would spend this much roster space on below-average quarterbacks when they are thin at other positions.
It is unlikely they keep all three the whole season and it may not even last beyond this week. Kizer is a guy that Mike Mayock loved when he came out of Notre Dame, and when all is said and done, it could just be Carr and Kizer for the majority of the season.
At the moment, the depth at the position is strictly based on quantity and not quality.