PostgreSQL hot standby promotion, multiple hot standbys
I don't think copying pg_xlog after you've lost the master is going to allow you to make ServerB or ServerC the new master.  If you might need to remaster, you really need to add log-shipping archives as well (this is a current limitation in Postgres, at least through 9.2).

Also, if the master has been shut down cleanly, you don't need to copy anything.