Quick question for LHR experts - AA is requiring us to checkin with an AA agent - are there any at a transit desk or do we have to go to the AA ticket counter landside? I am not sure if BA can help as the BA counter could not issue BPs for the connecting AA flight

we are arriving at T5 on a BA flight and have to transfer to T3. I guess worst case is we go out land side and transfer bus to T3. We have 3 hours