Next year projection:
Carlson: 38-45
Shattenkirk: 40-47
JJohnson: 50-55
Without your team and league details it's difficult to give you the right advise.
If you need a goalie or a forward more than a D, shop JJohnson and see what you can get for him. If not, keep him. He will help you to challenge again for the 1st place next year more than Shattenkirk and Carlson.