When I was switching from pick playing to two finger playing my teacher gave me some exercises to get both fingers strong and consistent - it should work for three fingers also.
1) Practice slow
2) Practice striking one string with one finger at a time. Make sure you get the same tone from each finger
3) Alternate three fingers on one string and try to make all three hits even with each other
4) Alternate three fingers on one string, vary your accents so that they fall on the first finger for a while, then the second finger for a while, and then the third finger. This way you learn to control where the accent lies and all three fingers are capable of playing a note unaccented or accented
5) Introduce string skipping up one string then up two strings, then up three strings, then repeat with going down strings.
There's lots of other things you can do in here but this gives you some ideas.
I worked hard. Anyone who works as hard as I did can achieve the same results. - J.S. Bach
CLUBS: Modulus #97/Gallien Krueger #957