I'm sorry, but luxury is relative to class. If you think a 6 series is luxury, but a 3 series is not, .... what is your definition, exactly? A 3 series is luxury because its compared to smaller generic cars, where a 6 series is ALSO luxury because it's compared to larger generic cars.
I'd never call a Porsche a luxury car because I consider them sports cars, even though many of their model lines are not particularly sporty anymore. Bentley/Rolls/etc are in a whole entirely different splashy show-off league where even many billionaires wouldn't buy one, simply because they're too ostentatious.
What's hilarious here is that you call the CLA (which is a bargain basement car) luxury, while a 3 series is not. Seriously, compare them. The 3er is a larger car and it's nicer. IF you're saying a CLA is luxury "in that class", then so is a 3 series.
But my Golf is nicer than a CLA. Of course, it's not considered a luxury car either, which is part of the appeal :)