I have a vertical and inverted single M3 as center and am real happy with it's presentation.

....I would also like a second one!

But yup; stuck in the middle of two more M3s as mains is a great solution.

And if for some reason your room should grow and you decide on a pair of M50s in the future; the M3 is still a perfect center.

Works great in my system! \:D