The reason there are destinations out of T3 isn't because the 757s "can't" be used in T5. They can be, it is just complicated and awkward to do so.

The reason there are BA routes out of T3 is for the pure and simple reason that T5 isn't big enough for the combined BA operation. It just suited BA nicely to partition T3 off with a dedicated fleet of 757s that came nowhere near T5.

With the 757s leaving the fleet, the main short haul fleet of Airbuses will serve both T5 and T3.