I, too, had a grade 3 AC separation (2005). I didn't go the surgical route and quit lifting for a few years because of frustration and pain. I started back up about 18 months ago and just took it extremely slow. I had to swallow my pride and focus, specifically, on proper form above all else; that meant making SLOW bumps in weight. With bench, I could only go as deep as my arms being parallel with the floor. It probably took 8+ months before I slowly started going deeper. Now I'm able to go down to chest depth and most of the pain is gone. I don't think I'll ever lift 100% pain free, but it's really close these days.
King Kull is spot on. Good warm-ups are so very important. If you have access to PT, you should really consider that. I wish I would have. Good luck to you. Remember, you're the best at understanding what you're body is trying to say, so be sure you're listening. I wish you the best and a quick recovery. Reality is that you're going to have to be very patient.