Put down a soldier and a harrier. Name the harrier: plane
Then put down a trigger. Fill in the boxes like this:
Activation: Radio Juliet, Repeatly
Type: switch
Name: bomb_refill
Text: Refill
Condition: this
On Activation:
plane addMagazine "Bo_GBU12_LGB"; plane addMagazine "Bo_GBU12_LGB"; plane addMagazine "Bo_GBU12_LGB";
With this you should be able to use the radio (press 0-0-0) to refill, you can even do it multiple times because you checked "repeatly". This is not tested though, sorry that I haven't provided a working example, but I don't have an ArmA to play with (patching).