I resisted trying this for a long time, because I really love my nose anchor, and couldn't figure out how to use it with the fixed crawl. Then I realized two things that made me decide to try it: first, I could just anchor further up my nose with the feather while using the same anchor for my thumb; second, I could just put a second nock right above my regular upper nock, so I would end up with two double nocks using only three tied on nocks. I didn't know if either of these things would actually work, but the simplicity was intriguing enough to make me at least try it. So today I tied on another nock and experimented with a higher nose anchor that I could reliably draw to.
The results exceeded my expectations. I ended up with a point on of about 22 yards, compared with my normal 45 yard point on. As soon as I determined this, my accuracy on 15-25 yard shots increased significantly. I am definitely interested in continuing to experiment with this. I will use my normal nocking point and anchor for 35-50 yard shots, and the fixed crawl for 15-25 yard shots. I have to decide what to do about 30 yard shots, which are sort of on the edge either way.
Why don't more people use this? Aside from the rules in certain tournaments, I suspect the main reason is the added complexity. Now I have to move two things: my nock point and my nose anchor, when I want to use the fixed crawl. I remember when I first switched to 3 under, and shot over the first deer I shot at after switching. Afterwards, I realized that in the excitement I must have used a split grip, since that was what I was used to. In experimenting with this, I found myself many times nocking in my regular spot and having to correct myself, even though I was in my own backyard and under no pressure. If I find myself doing this when the chips are down, I will probably have to rethink this.