Here's a book title that might possibly help:
How to Improve Your Marriage Without Talking About It
... by Dr. Patricia Love and Dr. Steven Stosny
It looks like it gets a pretty high rating.
When you do "talk about it," you might want to research something called "Nonviolent Communication." It's a style of communication developed by Marshall Rosenberg. From what I've heard, it helps reduce some of the "drama" that M so dreads. A couple of links regarding the subject are:
Having a reluctant communicator in a poly relationship is certainly a challenge, but not an insurmountable one. I myself am one of the guys in a V where the other guy pretty much hates communicating, and shuts down and goes into "silent mode" when something is bugging him or getting him down. There seems to be a couple of ways we approach this. One is to just give him some room and let him figure stuff out, and he'll usually talk eventually. If that doesn't work, gentle verbal nudging will often get him to talk in very small bits and pieces. But it takes a lot of patience.
Communication comes more easily/naturally to some people than it does others (just as some people have more struggles with jealousy than others). It takes a certain amount of acceptance to live, deal, and work with a "challenged communicator." You have to realize that they'll probably never be a great communicator, but there is a chance they can improve little by little over time.
In the meantime, check out that book and those links and see what might help in the short term.