OBDII Code P0420 BMW - Catalytic Converter Conversion | Engine-Codes.com

OBDII Code P0430 BMW - Catalytic Converter Conversion 2 | Engine-Codes.com

Yeah, doesn't seem like it would be the O2 sensors, according to those links.

How many miles on your X3? I'm not a mechanic or anything, but I'd probably start with any deferred maintenance, like spark plugs or injector cleaner. If that doesn't work, you might need a new cat.