Hi Gary, welcome.
This topic has been hashed through here a number of times. If you do a search for relevant terms, you'll probably find a number of people shooting off their mouths, er, offering opinions.
I run 5.1, so take this with a grain of salt. To answer your question directly, it depends somewhat upon your room configuration and size. Most people would probably assert that the M2 is plenty for the rear surrounds and also a better tonal match for the M60's than is the M3.
I think you'd probably want a fair amount of space behind the listening position in order to fully appreciate direct-radiating speakers as rear surrounds.
