Who is the third pilot in this Qantas Airbus 380 long haul flight?
The two stripes on his shoulders indicate he is a Second Officer. They are fully qualified pilots but only act as cruise relief pilots on long flights. New Qantas pilots are  initially employed as a Second Officer. 

It is standard practice to have all pilots in the cockpit for takeoff and landing. Their role in these periods is pretty much to just keep an eye on everything in case the other two miss something. Some time after the aircraft reaches cruising altitude one of the pilots will start their break and the second officer will take over.