You could use an M3 for a center too. Don't get too fixated on a "center" channel speaker design. The driver arrangement is kinda/sorta designed for center duty, but there is a bit of marketing BS with centers too. Center channels are generally designed in a horizontal configuration for placement considerations. But if you have room under the display for another M3 in the vertical position, and you enjoy your M3's, you'll enjoy another one in the middle.