yea, 3 notes per beat vs. per string, would be triplets. you cant really distinguish it "by string". you can have as many notes as you want "per string". for example, playing A Minor Scale, sometimes i play it in "triplets" meaning three notes per beat...but the scale is only notes per string....i could also play it in quarter notes, one beat per string.

make sense?

like they said above, each "beat" or "click" on the metronome equals a quarter note...4 quarter notes make up one measure. one measure is a whole note.

1 note per beat is a quarter note (4 quarter notes = one measure/whole note)
2 notes per beat is 1/8 note (8 1/8 notes = one measure/whole note)
3 notes per beat is triplets (12 triplets = one measure/whole note)
4 notes per beat is sixteenths (16 notes = one measure/whole note)