You're conflating wheel size and tire size. The wheels have their own specs (width,diameter,bolt pattern, and offset)...and the tires have their own specs (width, aspect ratio, and diameter).
The x3 wheels have specs that are compatible with the x5...but the x3 uses a different size
tire...you need to keep the x5 tire specs in this case.
A good 10 minutes on a site like tirerack.com can quickly educate you on the meaning of tire specs so that a situation like this won't throw you off base. They have great info on the meaning of tire specs...give the site a quick look.
You can see the "wheel" compatibility below...but again, tires are a separate entity to consider...they factor in load rating, speed, weight, suspension, etc. of the vehicle...that's why you'll see a recommended tire & wheel size chart in your owners manual & somewhere on your driver's door or door jamb:
