I'm sure Virgin will be pleased to be able to offer their own ground experience, upper class wing, etc to their passengers at T3.
I think the plan was for AA to move to T5 even before COVID, so I doubt that they will move back, there was some talk of an AA, new BA or dual-branded lounge at T5C and a refurbishment of the lounge complexes at T5, but that has all gone quiet. The other one world lounges at T3 were Qantas and Cathay. Qatar and Malaysia were at T4, no sign of that terminal opening, so quite a lot of displacement still, even with T3 reopening.