the offset of the x5 and x3 wheels is usually around the same. Depending on which, it might actually be exactly the same.

the difference is that the x5 uses tires with a higher aspect ratio for the sidewalls, so the wheels with the x5 sized tires will not clear the wheel walls in the x3.

if you just got the x5 rims, and mounted properly sized wheels onto them for the x3, they will fit.