I believe you will find the spare wheel hides under the car with a wind-down handle in the "tool-zone" under the boot/trunk floor panel.
Read the user manual to see the details.
BTW - I suggest you buy that "mechanic" a monkey-wrench since a monkey would have found the spare wheel under the car in no time at all. Additionally, any BMW mechanic should have known where the wheel might have been and even without having ever seen one he should have been able to recognise the round, tyre shaped item under the car behind the rear bumper. Bah-humbug..!
Good luck - your non-run-flat tyre must be trashed as well as your wheel rim. Ouch..$$$...!

Read your manual, it tells you exactly where the spare is :rolleyes: . There is a handle in the rear where all the tools are that will release the spare tire. You can't see it just by looking. I'm always baffled when people spend hours asking questions on a forum that can be answered in 30 seconds if they'd crack open the owner's manual.

If the sales rep at your dealer told you your X3 had run-flats, then he's a twit that should spend more time working on his product knowledge.

Well, Did you find the spare???:dunno:

On thursday when I went to pick it up, they made me pay 390$ for a new tire. Later on that evening they called me to admit it was their mistake and i was going to get all my money back.
End of the story. So I dont have runflats, they showed me where the spare is...