Mine holds the humidity very well !! However, I do not keep my humidity in the really high range. I use the coco bedding and mist it when I want to raise the humidity. I leave it around 45 or 50 and only raise it during shed. Works for me, never had a bad shed that way...
Glass tanks are horrible to keep humidity in, though, and if you plan on using one for a young hatchling you will be doing nonstop misting. If you use a glass tank, try to cover the sides, back and top, you can still see inside of it from the front, but the snake will feel so much better not being so "exposed".
3 ft. cage is definitely big enough to make a nice display setup for a male BP.