Just take any 3 flights with around 13h and similar ESAD, one 380, one 340-500, one 777W. Then compare them (weight to max adjusted) and determine the kgs of fuel used per kg payload.
The outcome turns around 2.1kgs - 380, 2.3kgs - 340 and 1.8kgs - 77W.
The eternal arguments that you can always go with max load on a 340, even though much lower, or you can carry 200 more souls on a 380 are both valid, but it does not bode away that the busses are less fuel efficient.
So it just comes down to the number of seats you can sell and the aircraft you have in your fleet. This tells you where to fly with what.
The 340s basically makes no sense no longer, but as no one buys them and you need capacity, use them where they lose the least.
The Airbus guys need some better routes and better rosters!