The basic idea is to have three rotating shafts that open or close holes that the balls can fall into. Previous attempts have been flimsy and not rigid enough, so I decided to create a proper solution. I use cheap servo motors for the rotation (the force required is quite small), but the small servo motors need proper mechanical fastening. I purchased some cheap ball bearings from aliexpress, and 3d-printed a servo holder with bearings to offload all force from the servo. Seems to work fine!