You can just switch iMessages off in phone C's settings. Or transfer the SIM from phone C into non-Apple phone.
Basically, the problem is that SMS-fallback doesn't work for group messages. So iPhone users can't send group messages to former iPhone-users.
It's not an SMS-delivery problem. Phone A never sends an SMS. Phone A sends only a "blue" message to the entire group, and never notices that phone C didn't receive it.
I switched away from IOS months ago, but my iPhone-using friends still see my number as "blue," and try to iMessage me. Individual messages fallback to SMS, but group messages just fail silently. There is no way to "sign out" of iMessage.