Don't think they're sold on Mannion.
And, you usually keep only 2 if you have a 3rd who is on the PS.
Problem for Rams is that the starter is a 2nd year player, his supposed back up is a 3rd year player, and they have no rookie QBs on their roster. So they'd have to take a PS eligible player that was cut by another team to place on their PS. How does he ever learn the offense? All the reps will of course go to Goff and Mannion.
What happens if Goff goes down for even 1 game? Mannion starts and then gets dinged for even a series. Who plays QB?
So, yeah, I expect them to keep 3 QBs. That 3rd QB is one heck of a lot more important than a 7th WR or 4th TE.
They can put some of their current WRs and TEs on the PS.