Hello! Do you have information on where the flat is located?

I live in the south of Brazil, and to us the expression "box triplo", or "box duplo" is pretty straightforward (not sure about the rest of the country, though). Usually, this kind of parking spaces ("box") refers to rectangles painted on the floor. If they had walls around we would refer to them as a "garage" parking space.

https://www.casamarimoveis.com.br/imovel/276-apartamento-3-d...

I hope that helps!