In regard to the counseling, we knew a therapist when we started out who was a friend of ours and we told her about our 'problem' and what we may do about it. After two weeks and some updates on our part, she told us that she would be out of clients if everyone would be like us. Meaning: we simply talked about everything among the three of us and it worked for us in such a way, that we were able to feel understood by the other(s), solved the problems at hand and were satisfied with the outcome.
I was worrying about the same things your wife seems to have in mind right now and the reassurence of my partners made me forget about it. It is terrifying to love this much and suddenly worry about the well being of not one but two people that are dear to you. I know the problem she has with guilt and insecurity in this sitaution. Everything is so new and one needs to get a grip on the things going on. This doesn't happen over night.
Most problems simply disappeared with time and good communication in our case. But if you feel the need to talk to a third, uninvolved party, go for the therapist sessions. I don't think it will hurt if you pick someone who is able to look at your situation from an unbiased point of view.
Facts: 30, female, bi, v-type relationship with Sward (husband, straight, mono) and Lin (boyfriend, straight, mono), poly-fi and co-primary.