Typically plucked bass is done with only I and M, but there aren't any rules. I've explored using three fingers, but I know I will have to put in lots and lots of time to be comfortable with it for (at least at this time) little gain in speed from just I/M. Look at players like Federico Malaman and Mohini Dei... I'd be happy to have have their speed and agility... and they do it with two fingers. But then again as mentioned, there is Billy Sheehan who uses three fingers. If you really want to go there his instructional stuff is probably where you want to start. I don't know if anyone in the bass or guitar world that uses the pinky for plucking (though there must be some). For me it would be totally impractical as my pinky is a full 1.25" shorter than my middle finger which is already 5/8" longer than my index.
Edit: I tried messing with multiple fingers and found that I like alternating I and A (ring) probably more than I and M... but would require retraining to do it as mindlessly as I use I/M. When using all three fingers I could do it, but couldn't find a pattern that was naturaly comfortable without considerable practice. The fingers seemed to pluck almost randomly when traveling across strings. However, when using the three fingered technique, while logic would hold that one would sequence I-M-A... I felt more comfortable with M-A-I (2-3-1, rather than 1-2-3). Perhaps it was the similarity of length of M and A, while my index is 5/8" shorter than M. Still... I couldn't comfortably alternate M and A, but yet could in fact alternate I and A. Go figure.