Do this regularly, most recently a few weeks ago, T5 (int'l.) to T3. Very easy (I do it landside), even more so if you only have hand luggage. At T5, go downstairs and take HEX one stop to the T1,2,3 station. Then walk the tunnels to T3. The worst part will be that the tunnels can be a bit dreary, but it's not really that bad at all.