The cases of the WSM's are very fat and you are probably bottoming out the follower spring against the floor plate thus making a very tight fit with three rounds in the magazine.

You can try to bend the bottom section of spring that attaches to the floor plate and straighten it out a little so it does not have the bow in it. That might give you the little bit of room you need for three cases. Or you can ask your dealer to try another follower that may be a little thinner than yours.