If I remember correctly, the Goldberg Cub has stripo ailerons. If that's the case with the ARF version, just cut the ailerons in half and put two servo mounts out in the wing panels for the ailerons and leave one servo in the center for flaps.

If the ARF has barn door type ailerons, it's gonna be a bit more difficult. You might even be better off just starting with a new wing kit.