You should be fine. The ride will be a bit firmer and the tires more expensive, but otherwise the X3 does fine with them. BMW actually sells 19" Accessory wheels.

Check with the offset before buying X5 wheels. Apparently there is a small difference in the offset, but I'm not sure if its negligible. Learned this when I tried to buy a set of X5 winter tires/wheels.

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2014 Mazda 3 GT: Ø Consumption: (click for details)