i saw that schools doing the B1 use the Daihatsu Cuore, which has 4 seats, so that wouldn't be a problem.
That's no guarantee that the WORD in Warsaw will use such a thing, though. You should contact them and find out what they're using for the test first, before signing up for any classes. Officially, you can only use a vehicle provided by you if it's for disability related reasons - so if they're conducting the B1 test using something unsuitable for 3 passengers, then you're stuffed.
Do you know any driving school that have sworn translators?
You don't need a sworn translator for lessons, just for the driving test.
How's your Polish?