Assuming it's all on time, that's tons of time, going landside at T3 should only add 15 minutes or so to the process if all goes well.

Follow the Flight Connections bus airside to T3, then when you get to the bus arrival area follows the yellow signs for Arrivals, through the e-gates and then you should be landside in a few minutes. The main departure area involves bearing rightwards (with M&S on your left), through some ill designed corridors and then you are in Departures (Arrivals is actually a separate building to Departures in T3 but they have inelegantly tried to join up the structures so that people don't realise it). Then Boots landside T3 is slightly tucked away up one floor from the check in desks, same level as security, so when you get to Departures use any lifts or stairway upwards. Boots is on the far right of the corridor facing security. Fast Track is by the Post Office, Boots would be to the right facing Fast Track.

You can go fully landside from T5 to T3 instead, but that would be slower.