Bloom, CC, and others will hopefully have a much more detailed and informed answer for you.
I don't know that you can assume a similar situation for these three guys. From my reading of the early scouting reports, these three are different enough that opportunity and scheme will play a big role.
Rufus Alexander sounds like a prototypical pursuit Will with some coverage ability. I've seen him listed anywhere from 220 to 230 pounds. Measured 6'0" and 227lb at the Senior Bowl weigh-in. Again, I'm no draftnik and the scouting reports I most rely on other than our own guys won't be published for awhile yet. But Alexander reads like a Demorrio Williams type player to me. Has great instincts and sideline-to-sideline range, but may not hold up well against blockers and needs to be in a scheme where he'll be protected. Sounds like a nice Tampa-2 candidate to me.
I can't get a hold on Lawrence Timmons from the scouting reports. It appears that some see him as a playmaking WLB with the ability to pursue and hit as well as an ability to get to the quarterback in pass rush. I've read reports that question his coverage skills, others suggest that as he matures he'll do fine. CC has posted elsewhere that he sees some Kamerion Wimbley in him. He may be a little small for a 'tweener OLB in my mind at 230 pounds or so. Some say he could end up being the most explosive backer in the class. I've seen him mocked to STL and PIT among others so who knows how he'll project in the boxscores. Does he grow into a Keith Bulluck, Karlos Dansby type player? Or does he end up as a Thomas Davis type SLB? Or is CC on the mark with a Wimbley/Lawson type comparison? I've no idea. Bloom has said earlier in the year that he'd be comfortable making him the prohibitive number one overall rookie IDP for now. If CC is right and he lands in a Wimbley-like situation, I don't think there's any chance he has that kind of ceiling.
Paul Posluszny is the best fundamental all-around backer of the three and certainly fits as a 4-3 OLB in any scheme. He's been taking snaps at WLB at the Senior Bowl after taking one for the team and moving inside at PSU. He gets high marks for his instincts and playing speed. But if he lands at WLB next to a solid MLB he may not have the big play capability to be a top IDP option.
Based on that, I don't know that you can clearly rank any of the three ahead of the others until we see where they fit. I'd guess Posluszny is the safest bet in January, but should Alexander land in Indy or Chicago or Timmons end up as a Giant with their new aggressive scheme while Posluszny becomes the Will someplace like STL (I'm just illustrating the point, I've no idea if any of those three will last that long and/or go that high) all bets have to be off.