OK. I am going to keep in mind that we're getting only your side of the story. Going with that... your wife is acting like a spoiled toddler. I get burnout, I get overwhelming responsibility, I get needing breaks, I get needing me time. I was a military wife for years and would go 6 months at a time with NO breaks from the children when my husband was out to sea. As a mom, you suck it up, find ways to cope, figure out how to have kid-free time and find ways to express yourself without pulling the chute and jumping out of the plane. It's called being a grown-up.
When you say the therapist said to have fun with the threesome... I'm getting the feeling perhaps your wife is not telling her therapist the whole story... or hasn't yet (it can take quite a few sessions to get into everything), including her wanting to run away and just be with T by herself...
I think couples counseling would be very valuable-- maybe with a different counselor though, it's usually best to not go to couples counseling with the same therapist that is one of the partner's personal therapist. She can continue to see hers, but I do think maybe you need to see one together so that both of your views/sides/issues can be addressed. The fact that she is being offered help (nanny/housekeeper, etc) and keeps saying no and then complaining to me seems like something deeper is going on. Maybe some sort of midlife crisis-- hate to be cliche-- but it does happen that sometimes the responsibility builds up and the person then feels like the ONLY solution is to leave it all behind.
Regardless, I think couples counseling will be helpful whether you decide to separate or not. If nothing else, they can help you and her clarify what's going on, and be a support person if you do decide to separate.