3 = ring
2 = middle
1 = index
DO NOT play 3-2-1-2. That *will* improve your speed slightly, because you relieve the amount of work on your index finger, instead trading off between that and your ring ringer, but the middle still has to work just as hard...every other note. So you don't gain very much by that, plus it's more work than it's worth to get the coordination for that.
People will try to tell you to do that for groups of 4 (like straight 16ths). Do not listen to them.
3-2-1-3-2-1 is the way to go. It's tough at first to get the accenting right for straight 16ths in groups of 4, but it doesn't take much work to get past the little mental block that's presented there. Mathematically (I can work it out some other time if you'd like) this way is MUCH more effective than 3-2-1-2.
Work on playing straight 4/4 with the grouping of 3, accenting every 4th note.
Also, you can use your index and ring finger instead of index and middle, because the anatomy of your hand allows for the first option to go longer without your forearm getting fatigued.