Streaming Replication Failover - how to point second slave at new master?
In PostgreSQL 9.2, you should be able to just repoint server C to B by changing it's recovery.conf file, if you are also using log archiving (and have a restore_command defined in your recovery.conf). It should work automatically - but it does require the log archive to work (machine B will send critical information about timeline switching to the log archive, which machine C will then replay).

PostgreSQL 9.3 will be able to deal with this repointing of the slave over pure streaming replication.