All 3 RBs have some injury history... and clearly, they want to improve their depth and maybe they don't trust C-Mike (some have suggested it... though I think that's mainly because after kicking him out the second time, Seahawks coaches seemed to suggest they didn't trust him, because he still hadn't learned stuff (rumored to be the pass blocking assignments which had the OL coach upset)).
But they're all different.
The first RB Williams is a nice power bull RB which some short area quickness... like a poor man's Eddie Lacy. And I don't love his skill set, but might be able to get away with it, because he's such a decisive runner (which I LOVE)... he's very quick to pick a pretty good hole and hits it hard and quick.
Then RB Jones is sort of a jack of all trades, seems to be able to do everything well, but doesn't do any one thing great. And he's 5'9" and over 205 lbs... so I can't call him small... short maybe, but he's got some weight... he's not one of the sub 195 lb RBs.
The third one, RB Mays, might have the most physical potential, as he's 230lbs but still has some nice quickness and speed... but he basically missed all of last year due to an ankle injury... some have suggested he would of been a 4th round pick if he came out after his Jr year.... but he stayed and injured his ankle.