Ideally, the tweeters of all three M22s would be at ear level. Obviously, you have to work with what you have. How would the center M22 be accommodated in your system? I don't have three M22s, but I have two M80s and an M22 which I have on a stand directly in front of my TV stand. This puts the speaker as high as possible without intruding on the picture. It also puts the M22's tweeter in line with the lower tweeters on the M80s. This seems to work well for me.

One option, if the right and left M22s are above ear level, is to turn them upside-down to get the tweeters closer to the tweeter of the center speaker, assuming placement of the center below the TV. I personally haven't had any luck placing my centers above my TV, but I assume that's simply because I sit so close to my screen, putting it at an extreme angle.