You said Lowrie would be average/above average. Above average 3B this year are from .750 OPS to .800 OPS and the top tier is from .800 to .920, although most of those guys are close to .850. Based on the samples we have, I don't think its unreasonable to believe Lowrie can't stay around low .800s considering that he's still a young guy who has been playing through injuries. Mark Reynolds is .818 for his career-- Young is .790, and Aramis is .838.
Lowrie's stat line has suffered badly from his wrist injury, and its pretty clear if you look at befores/afters for the 2008 season, but he still has a .740 OPS. I don't believe he's going to stay healthy... but if this team needs to make a sacrifice at one position, which duo would you rather see starting-- Lowrie+Crawford or Beltre+Cameron?