I am backing up my own laptop to two different masters. Only one can be the Master, but the other one is simply listed as another Server. When I want to test user-initiated backups and restores to/from my lab environment, I simply add -M <lab-master> to the command line. Scheduled backups from my lab master also work 100%, no configuration change needed.
I do not recommend trying to do backups simultaneously, especially if client C is a Windows system and need to take snapshots for open file backups.
If you have any database agents, it will ONLY be sent to the Master (1st server entry).