BzzzzzT
100+ Head-Fier
The pin-out for the C3m is different than the C3g, and thus, a different adapter would be necessary. So yes, the C3m will work using the 408A heater settings, but you can't use the C3g adapters. However you can always ask one of the eBay vendors to build adapters for you, or if you are handy, perhaps you could build your own.
I tried some C3ms. They sounded good but not as good as the C3Gs IMHO. The real problem was it made the 10ohm (ver 2.0) or 20ohm resistors (ver 3.0) way too hot. They would get to scalding temperatures in around 15 seconds. With the 408A tubes, the resistors get hot but not scalding hot.