When using my fingers to pick the strings of my bass, which I don't do all that often more, since I much prefer the aggressive sort of tone with a lot of emphasis on the initial attack a pick gives me, I use a combination of just about any technique there is, double thumbing, holding the index and thumb together using the edge/nail of the index finger as I would a pick, classical acoustic guitar finger picking, as well as, for bass, more traditional 2 and 3 finger picking.
Not sure if that covers how Gary Willis in particular does it, not really familiar with him, but yes, when using my fingers to plug the strings of my bass I sometimes will use 3 fingers.