Grannyring good to hear you are about to get them;My m2's are 3 ft off the backwall,8ft apart inside edge to inside edge,slight toe in, and I listen 10 ft from the halfway point between the panels.
I think Duke is correct with getting the panels off of the backwall by 3.5 or more is even better than what I have.
Duke also mentioned the ficus tree to diffuse the rear wave or using a sallie;I asked a question a long while ago on which should be used Diffusion or Adsorption and Rives Audio responded to use Diffusion;and I agree 100% on this issue as I tried both.
I think most important is to not rush the process in the setup procedure;your ears will tell you when it is right.
On using a sub I think your M3's will surprise you with the accurate bass reproduction they are capable of.