Asap, the best "match" for reproducing the center channel is likely to no center speaker at all, but rather the left and right speakers handling the job in "phantom" mode. The disadvantage of this is that there is a relatively narrow horizontal area in which the center channel material appears to come from the screen. If this isn't acceptable, then the best match would be a 3rd speaker identical to the left and right and positioned vertically above or below the screen.
Since in your other thread you say that you'll be using a sub, the small advantage of the M3 in the bass shouldn't be significant to you. Because of the similarity of the new M2i to the M22, I would suspect that, if anything, it might have a bit stronger mid-range than the slightly recessed M3( note that SoundStage has a review of the M2i planned, but there's no indication as to the date). Since the old M2 is being phased out, the factory outlet price is indeed attractive, but I suspect that you may be happier with the M2i(have you read Ian's discussion of it in the June newsletter in the archives?). In short, my suggestion would be that you consider 3 M2is.
Enjoy the music, not the equipment.