HannahB,
I'm sort of at the same point you are. My backwards skating on 2 feet is OK, I can swizzle and also shift weight back and forth as long as 2 feet are always on the ice. Shifting weight and picking one foot up is just scary, and I can do it (like you) for a fraction of a second. I can hold it longer if I'm holding Coach's fingertips, and she swears I'm not using for balance, it's just a confidence thing.
Regarding 3-turns, I can tell you that I started learning 3-turns way before I started learning 1-foot backwards glides. Coach had me learning the toe loop jump and Salchow jump, both of which involve a 3-turn before the jump (toe loop uses a right inside edge 3-turn, Salchow uses a left outside 3-turn, assuming you jump counter clockwise). We used the wall to do the 3-turn part and then the jump. So I just wanted to let you know, don't stress out about not being able to hold a backwards 1-foot glide before learning a 3-turn...you can use the wall for support while learning the 3-turn, even if you can't hold the glide after turning.
Actually I can hold a backwards 1-foot glide when I'm coming out of a jump like that, much better than I can from just a 2-foot backward glide...probably because my weight is most definitely over my landing foot, I don't have to "shift" my weight. I am still doing these jumps by the wall, although I'm only lightly touching the wall. So to practice backwards 1-foot glides, what I try to do is: Do one of those jumps by the wall, landing on my right foot and holding that backwards glide as long as possible...then put my left foot down, do one backwards swizzle, then try shifting weight to pick up my right foot. That way, I've already got some speed going backwards, plus a very obvious weight shift already there.
Keep practicing!
Rene