I'd like to see them pick up a power RB in R4 or 5. Aside from short yardage and goalline situations, I'm concerned that until the Bills establish a deep threat, defenses are going to play them pretty tight and there's not going to be a lot of running room.
Depth on both lines is always useful on day 3. So are DBs. A decent C prospect, in particular, would be a nice addition.
Another WR with some downfield speed is a big need, but I'd rather have gotten a more complete player than what is likely to be left.