Because of the carbon footprint of Jaggar's solution (total fuel consumption in his scheme is 3 + 1 + 2 + whatever, I refuse to do the fractions...), I think a solution with less fuel is preferable. Thanks to my colleague Nick D:

Planes A, B and C take off west.

After 1/4 tank has been spent, C dumps 1/4 tank in each of A & B and returns on the remaining 1/4.

After another 1/4 tank A and B have each 3/4 left. B dumps 1/4 in A and returns on the remaining 1/2.

After A has spent another 1/2 tank, he is half-empty on the half-way point of the trip. B refuels and takes off east.

When A is empty, B still has 1/2 tank left which he shares with A. C takes off east with full tank.

When A & B are empty, they just meet C who has 3/4 tank full and they are 1/4 tank away from home. C shares with them and they just make it home.

Empty tanks but total fuel consumption of a mere 5 tanks full.