Every carseat I had was totally incompatable with my 1987 BMW325, but that's a totally different car

alternatively,
Britax seats fit the 2002-2004 BMW 5 series really well

Sadly, I haven't seen an X3 in these parts

However, the database within this site shows that
Britax does fit the X3
How you tried installing it yourself? I'm only 100# and rarely need help

Make sure to thoroughly read both vehicle + carseat manuals 1st & post any questions here.... Do make an appointment for your local tech, too, though.