Your oem 235/55/17s have a rolling diametre of 690mm,the 245/35/20s of the 645csi have a rolling diam. of 680mm, a difference of only 1.44%!
In comparision the X5's 315/35/20 is 729mm!........+7.20%).

These tyres should fit your X3;you will just be riding 10mm lower than standard!(1.44% better accelleration,less top speed)

Just checked Hartge's wheels/tyres for the X3 and coincidentally they used
Front:245/35/20
Rear:275/30/20

So,if price is right,go ahead buy them..........