If you want to be able to play fast runs then also practice four note per string scales. You will basically get 6 extra notes in over a run covering all 6 strings. However, the scales will climb the fretboard diagonally which can be tricky if you are a beginner. I would recommend you practice 4 note per strings scales along side the 3 notes ones as it will give you a good technical grounding from day one. One other thing to watch for is the difference in picking from 3 to 4 notes per string. This is one wasy to master alternate picking, which is probably the most important technique of all. Good luck with it.