If you wire it like the OBP-3SK/PP, then no drilling is needed. The OBP-3SK/PP has the bass and treble on a stacked pot; the mid is on its own push/pull pot. This would leave your third hole open for a stacked master volume/blend pot, which you already have if you still have the Thumb's original electronics.
Edit: I forgot that the Thumb BO does not have a stacked master volume/blend pot. Only the NT has that setup. So, you'd have to purchase a stacked pot that you can use for this function.