KR is anyway NOT a reserved ST position. I believe when the OP said 3; he meant: K, P, LS.
KR can be a [second job for a] CB, S, RB, WR.
In today’s NFL I believe PR is more likely to be a WR, CB, S
and the KR is more likely RB ( as you suggest) or a S, stout WR, or maybe even think about a crazy thought: TE.
But in no way will or should most teams hold a player on roster who is either KR/PR -only. BB was an outlier keeping Gunner-only players rostered. I would still see a Gunner as much more likely than a Returner to get such special accommodation.