It's not a good idea to use different tyres front to rear. They may be the same nominal size but may be different in actual size. They may also have different grip characteristics. I'm not sure what the X3 owner's manual says but I know that the X5 manual stresses the important of using the same brand/size etc on all four wheels. You have a nice vehicle ... don't compromise it by mixing up the tyres. There may be some people here who have done this and will say "I've had no problems", but I think it's a dud idea.