Hi there,
I have a Maxpedition universal sheath, and I would say that it is more suited to open view duty belt carry, due to the bulky nature of the pouch and it's belt attachment fittings.
It will 'fit', and can probably be made to fit quite a few lights, as it's velcro adjustable all over the place. With the M3, in order to have the grip ring slide into the pouch, the pouch needs to be set up quite a bit wider than a plain tube light such as a 9P, this results in the pouch mouth being wider than the body tube just below the head, and the light wobbles in the pouch a little. I have found, that the SureFire V40 pouch (for the 9AX) will fit, even with the long Z48 tailcap fitted, you may want to look at this option, it's cheap enough. I am waiting for a BDS M3 pouch to arrive, but can't comment on this yet, obviously. I'm at work tonight, but I will try to get some photos posted tomorrow of my pouch options for the M3, it's an odd shaped light to accommodate !