I use my three fingers (index, middle, and ring). The secret of tremolo is the way you rotate your fingers.

for now: Index = 1, Middle = 2, Ring = 3.

I start off with the index finger and move like this:
1321321321321321321321321

So basically I gallop with me three fingers, but since I start with my index and then jump to the ring, the tremolo doenst sound like a gallop.

At first, I dont want to use my thumb, but just make sure my tremolo fingers are tight and fit. This constant rotation will cause fatigue on your fingers and forearm, so expect to practice this daily until you get a slight pain ---then stop, dont practice on constant pain. Rest for 15 minuets and start again.

You want to practice this on the high E string and once you got it good at speed then start adding the bass with the thumb.

The only part I dont understand you is the 3, 4 string deal. What do you mean? Can you give me an example on a tab? Are you sure your not talking about arpeggio?

oh and BTW, it didnt take me 3 years to learn it. It took me much shorter, but once you reach a platuea, you can go any faster, better, or more accurate. But I was just saying withing my 3 years of playing, I can temolo quite accurate.

No person can really say how long it will take them to learn a technique, its a on goin practice and you can really put a timer on it. Its like asking how someone learned how to bend. A person playing for 3 weeks can bend just as far as BB KING, but I doubt they are on par with eachother...