I don't think they will match well, but since they are just surrounds, it probably won't be too bad for movies. For multi channel music, it will be noticable. Front soundstage sounds good, but I would say just get another pair of M3s for rears.
John is right I have M22s up front and Technics Floorstanders for my surrounds. They sound fine during most movies but if there is identical music coming from all the speakers you can tell the difference. Its subtle though and none of my family and friends could notice it. If youre gonna use your system for the new multichannel formats (SACD or DVD-Audio) I would want matching all the way around. Never tried my setup like that though so I'm not speaking from experience.