Bentley produces a comprehensive repair manual for the E53 X5, but has not yet released one for the X3. Here is a link to DVD's that can be found on eBay that claim to be X3 repair manuals. I have no experience with these, so I can't tell you whether they are good or bad. But the prices are low, so you can't go too wrong by giving it a shot.
http://shop.ebay.com/items/_W0QQ_nkw...Q_fromZQQ_mdoZ