No one seems to be chiming in here - there are some good instructions for doing this on the X5 (where it seems to be more of a common problem). Have you checked out :
http://www.x5world.com/articles/x5/e...nt-bmw-x5.html
The X3 should be somewhat similar, and hopefully a touch easier, especially the rears. But usual precautions pertain on avoiding damage to electrical components, watching out for heavy glass falling on your hand, and preserving the waterproofing when reassembling. Can't see that the glass would have become totally inoperative without needing a regulator replacement, which gets a bit challenging based on other vehicles in my experience. Good luck.