If you can fit a third M3 as a center it will be the best possible match to your M3 mains and cost less than the horizontal alternatives. However, remember the M3 is rear ported so if you will have to put it inside a cabinet you might consider a VP100 or VP 150 which are not ported.

3M80 2M22 6QS8 2M2 1EP500 Sony BDP-S590 Panny-7000 Onkyo-3007 Carada-134 Xbox Buttkicker AS-EQ1