Look in Germany, 3-er is just how they roll; it's not wrong, it's just their way of short hand.
Cars has their own slang everywhere and 3er is completely normal in Germany, it's all over BMW's German website (https://www.bmw.de/de/neufahrzeuge/3er.html), and on enthusiast forums.
3 series is Drei-er or Drei-serie, with the Drei being 3.

We have German specificity to thank for Sixt's insistence on BMWs and specific models so relax, grab a pretzel, and make sure you're pronouncing Porsche correctly while you're at it

Tbh I’d prefer getting more specific: which 2er and whether the SUVs LCI to know whether it had adaptive cruise so let’s count the general forum’s blessings that OP didn’t say G60 LCI, G20 LCI, G22 pre-LCI, G42/F74 pre-LCI to specify what’s in the fleet.

3 series wise, I’ve walked up to an LCI F30 instead of the G20 I was expecting at Hertz, was NOT a good day.