T5-T3 transfer & checkin should not be more than 45 mins, then allow 20 for security and then 5-15 mins to walk to your T3 gate, all up about 90 mins max.
Don't worry about the displays in T3 food court/ shopping centre not showing a gate, ask at check-in or go find it yourself, and do some spotting depending on the gate, or find one with a view.
T3 at LHR is a bit like Ryanair, play by thier rules and you will be fine.
Anthony T