Rik,
Which style X5 19s do you have?
Not that it matters; the offset on X5 wheels is different,
but I've seen pics here of X5 wheels, (even style 87 20s),
on an X3. The increased offset of most X5 wheels, (vs most
X3 wheels), may promote more tire wear, but worth a try,
though not sure if any full size wheel spare is going to fit
in your X3's well.
Here's a couple links for wheel sizing: first is a list of BMW wheels
with pics and "sizing", inc. width and offset; use the drop down
tab and then click on your two wheels and note "size".
http://felgenkatalog.auto-treff.com/
The 2nd link is a powerful graphic comparsion that allows for
exact wheel/tire combos and show the end view of wheel/tire
position and hub set.
http://www.bigcustomwheels.com/rt_specs.jsp
Another angle may be to cop a used or non-oem wheel/tire,
in your X3s size,
if it fits in your spare area, for back up,
giving up cosmetics.
GL,mD