If you're using 3 fingers, try assigning 1 finger per string. You can start practicing this with single octaves, using your first finger for the root and the ring finger for the octave. This will require you to rotate your right hand slightly everytime you want to reach for a higher string, but it becomes natural with practice.
A particular exercise is grabbing a root note and start playing 8ths at midtempo (around 140 bpm), playing root, octave, root, fifth and repeating; then 2 roots, 2 octaves, 2 roots, 2 fifths and repeating; and then root, 2 octaves, 2 roots and 1 fifth. Increase the tempo and shuffle the patterns as you get more precise, always using this finger per string approach. Also, don't stay in only 1 position, start shuffling or apply it to some chord changes (when you get faster, you'll get this cool disco feel as a byproduct).
In the long run, if you get better, you'll find that it's cleaner than raking and more natural than alternating 2 fingers.
Hope it helps!