I have 3 in a 2.5 mini-bow (just got the third one to replace one that died right after we got them originally). Mine have never tried to jump out that I know of, but I read that it is a possibility. The lid on my mini-bow has a big open section in the back for a filter or whatever. I have covered the opening with foil until I find a more permanent solution because I don't want to take any chances.
Keeping the water lower is a very good idea. Even in a 2.5 one of my frogs couldn't get to the top at first. He just kept trying and trying, but each time he'd get 1/2 to 3/4 of the way to the top and despite his best efforts, he'd end up floating back to the bottom. With the water level lower none of them have any problems now.
Also, when you go get it/them, after you've looked to make sure it has webbed feet, etc., make sure they catch ones that look healthy (i.e., good color, all legs working properly, no visible sores, etc.). I know this sounds silly - of course we try to get healthy animals, but I've seen some pretty sad looking froggies at various places.