It's orthogonal to the M+S rating. You can get a 3PMSF stamp without M+S.
Sure, though I think it's safe to say it is strictly better. M+S is a geometric tread definition, and such tires may have awful compounds that get rock hard in snow. 3PMSF requires a tested performance level in order to achieve. The only time I can imagine an M+S being better by any metric is if that metric is not related to cold performance. E.g. treadwear, price, warm weather handling, etc.