I think Qatar might refuse, they're really, really not going to be keen to be jammed into a Ryanair dominated T3, and that's no slight on FR who I have flown happily many times. T3 has major pain points in boarding, especially around Gate 44, it was a cul de sac of mayhem some days when American used to board from 44 or 48. You're not going to want a Qatar B77W or a Cathay A350 in that mix. The customer experience is overwhelmingly loco and even BA don't belong there anymore.
When BA moved to LGW South, there was a financial inducement, pier one was brand new, the satellite is pretty spacious and they got a new lounge and dedicated large checkin area, and plane to train was way quicker. South lost easyJet and the loco vibe with only a few Ryanair, Vueling and Norwegain remaining. At MAN T3 you'd have a bussling FR dominated loco terminal with some Oneworld squatters IMHO.