3mm isn't material until it is. 3mm is what? about a 1/10 of an inch? Doesn't seem like much to worry about but I think the answer depends on the geometry and measurements of the CLA's brake assembly, suspension, existing space from the fender, etc. For example, how do the CLA45's calipers compare to the C63's -- are they exactly the same shape and size? I don't think you necessarily know the answer even if the wheels had the exact same offset. You need more information about the CLA45 to reach a conclusion -- everything you're going to get here is just a guess without more data.
Just my $.02,
-Eric
Just my $.02,
-Eric