There is no longer an indoor link between T3 and T1 to connect to the enclosed walkway from T1 to the Station.

The new arrangement is a covered walkway, but with open side, at ground level from T3 to T1 and thence to the enclosed walkway up at departures level.

The walkway is not well signed when you approach T1, but head for international arrivals in T1 and then look for signs to the Station.

The walking time is about 15 minutes, quicker if no luggage and a fast walker.

Alternatively there is an erratic bus service. The last few times I've given up waiting for it.