czarnykot:
Do multi-storey car parks exist in Poland?
They do.
czarnykot:
Może 'multi-storey car park' po polsku jest 'parking wielopiętrowy'?
I'd say 'parking wielopoziomowy' but your version seems ok too.
czarnykot:
Są trzy samochody na parkingu
Correct. Or maybe "Na parkingu są trzy samochody" sounds a bit more Polish... Depends on what you want to stress - the cars or the car park.
czarnykot:
would 'in a multi-storey car park' use ' w + Locative case'?
No. Saying 'w parkingu' sounds clumsy and unnatural. 'Na parkingu' is the correct version, no matter if it's an enclosed or unenclosed space.