Rev Gary Davis was another thumb and index finger player.

Gordon Giltrap has an unusual style. He uses a plectrum and his little finger only.

Personally, I’m a thumbpick with index, middle and ring finger picker, with the little finger planted on the soundboard.

Sorry if already mentioned, but the Africans are usually quite insistent on the use of only thumb and first finger. I once read an interview with Jean Bosco Mwenda (most famous tune 'Masanga'), and he described the importance of not using the other fingers as there's a more consistent tone just using thumb and index.

I started using thumb and three fingers. Then I had some lessons with a blue fingerpicker, and he mentioned I would probably be more agile using just two fingers.

For me, he was right. My ring finger follows my middle finger a bit and isn't so independent or agile. My index and middle fingers move independently and are strong. So now I only use the ring finger when I need to, otherwise it is just thumb, index, and middle.