Afraid my advice probably won't be much help - what ever you want to do.
You will submit 3 ideas, I think. Shoot, I'm still in the course and am forgetting the beginning. Or there is also the picture book option (and nonfiction option). For what I submitted, I submitted one idea that was just a spark on an idea, one that I had the basic plot and first two chapters written, and one that I had a VERY rough draft and lots of other ideas. The instructor felt my last two ideas would better serve my time since I had at least a basic plot for both and she felt the final idea was the most marketable. The instructor will guide you as far as what he/she thinks will work best, but the final decision is yours. I decided to work on the last idea as it was a story near and dear to my heart and one I've worked on for years, but have never been able to finish. I actually ended up scrapping A LOT of it except for a few scenes here and there. I rewrote a lot of the first half from scratch just pulling dialogue from the original draft to fit in the new scenes. But I knew my characters completely so didn't really need to worry too much about character development. The basic plot idea stayed the same, but I was able to figure out what was working and what wasn't.