Did you look at the Sticky on the top of this sub-forum? There is an excellent chart that lists every tire size that is compatible with the CX-3. It also includes the % of difference from the stock tires.
The existing GT 18" rims will easily accommodate a 225 or 235 width tire. Both tire sizes fit onto the rims and will not rub on anything. However, depending on the aspect ratio you go with the tire circumference will be larger, or smaller than the stock tires and this will affect your speedometer and odometer. A lower profile will also give you a harsher ride.