Airside lounges at LHR are designated departure lounges, so it’d be unusual to be allowed to return to use them if you’re connecting to a flight departing from Terminal 5.
Terminal 3 lounges include:
Airside, departure lounges:
American Airlines Flagship / First Class Lounge - Lounge Zone H
American Airlines Admirals Club - Lounge Zone H
British Airways First and Club / Business lounges - Lounge Zone F
Cathay Pacific First and Business lounges near Gate 11
Qantas Airways London Lounge - Lounge B / Gate
Landside, for arriving premium passengers and Emeralds (no guests)
American Airlines Arrivals Lounge (recently renovated)
Terminal 5 lounges:
Airside lounges:
British Airways Concorde Lounge (longhaul BA First)
British Airways Galleries First Class Lounge - South Concourse
British Airways Galleries Club / Business lounges both North & South Concourses
British Airways Galleries Club / Business lounge T5B Satellite
Landside, for arriving premium passengers and Emeralds (no guests)
British Airways Arrivals Lounge (recently renovated)
For hours, access, amenities, etc. consult the
oneworld lounge access tool
here.
For transferring T3 <-> T5, see
https://www.flyertalk.com/forum/amer...onnection.html.