Fuel transfers to the trim tank for cruise CG control its a rather sensitive (and pretty unreliable) system which doesnt allow for long range flights when inoperative. Some A300s didnt come with it actually, but all the A310-300s did. A310-200s did not. Anyway, when it transfers back to the center tank around 2200-2500 remains in the center and is not used. When you account the fuel in the manifold, it accounts for nearly 3000 lbs.
Fuel is transferred by ground personnel. Its basically turning the center fuel tank pumps on and transferring the fuel to the wing tanks from the refuel panel below the right wing.