Originally Posted by
flatlander
You don't have to go outside at T3. There is an inconspicuous path between the Caffe Nero and the lifts in the arrivals area. On arrival, go towards the landside Caffe Nero and keep going past it and you'll be in the landside checkin area.
The route between Caffe Nero and the lifts doesn't lead anywhere other than upstairs, which takes you to the the short stay car park and the balcony above landside arrivals. You can access the departures building via the balcony.
The route I think you're thinking of is at the other end of the landside arrivals hall, adjacent to the toilets. There's a cut-through there to check-in Zone G.