Taking a flight LHR (terminal 5) to AMS on same plane back to Terminal 5 LHR. The long flight in the night is from LHR terminal 3. Could we check in the luggage in terminal 5 for later travel from Terminal 3? Or should we go and check in at terminal 3 and then go to 5 for the AMS round trip?

On return from AMS to terminal 5, should the flight connections be followed or come out and go land side to terminal 3. There is plenty of time before the onward flight.