For QBs, I recommend Sam Bradford, Kevin Kolb, Matthew Stafford, or Colt McCoy (if you're in a gambling mood). These four are usually not taken, and each one has a bright upside for the future. However, each one has some risks, so consider what you want to do. Obviously if you can get your hands on someone like Rodgers or Brees that would be best, but those three should give you a decent shot and are loaded with potential. Sometimes Eli Manning and Jay Cutler can be found in the later rounds too - it all depends on how many people are in the league.
For RB, I would consider Mark Ingram, LeGarrette Blount, Felix Jones or Fred Jackson. Ingram may not do much for you this year, but he has the potential to be a big player for you in the future. Blount, Jones, and Jackson are all good, solid runningbacks who tend to go late and should be available. Jones is having an especially good preseason. Avoid Marshawn Lynch if at all possible.
For WR, look at Julio Jones and AJ Green. They're rookies with promising futures, and Jones has a good chance to be productive right away since he's a part of a great offense already (Atlanta). You could also look to guys such as Mike Williams (from Tampa Bay), Santonio Holmes, or Lance Moore. There are quite a few good wide receivers in the late rounds, so you should be able to find someone serviceable.