The MS3 is designed to clip to an attachment point on the receiver backplate to take proper advantage of the 1 to 2 point transitions, and the CTR stock has slits in it for traditional 2 point slings. They weren't really meant to be compatible as I don't think anyone thought that someone would ever want to attach them to each other.
So you might have to improvise. The CTR stock has QD points built into it, so you can plug in something like this:
http://www.wolverinesupplies.com/de...ced-Heavy-Duty-Quick-Detach-Sling-Swivel.aspx