BA will not be vacating LHR T3 when T5C is open in a couple of months. BA has commercial reasons to maintain a presence in T3, not least after the big investment it has made in Galleries lounges in T3. I imagine GIB will continue to operate from T3 with T5C being used to reduce bussing to current T5 flights.
Once T5C is up and running BA plan to transfer GIB, LIS and VIE to T5 from T3. This was originally planned for MAY/JUN11. It is now thought this change will take place at the end of the S11 timetable
BA's flights to BKK, BCN, HEL, MAD, SIN and SYD will remain at T3 due to there codeshare agreements with AY, IB and QF.