I think the only way all three of those can leave is if we get to 10+ wins. If we lose to UGA, Bama, South Carolina/Florida, how is that going to show NFL scouts that those guys are ready for the next level? Notice that they guys that leave college early are generally on very successful college teams. Now, if we were to put up 5+ TDs per game and still lost some, that could be a different story, but not very likely I don't think.
All that to say, if we win that much, we should bring in one heck of a '13 class, likely with a few players ready to contribute right away, which should help. The O-line and running backs by next season should be substantially improved and experienced. And I think our D is going to be very good this season, and hopefully will continue to improve. If those three leave, it will still be more of a re-loading season, instead of re-building. Three players leaving won't take down the team.