It's asking for your parents' acquisition date and cost basis. Since it was a gift, your acquisition date is the date that your parents bough the property. Your cost basis is 1/3 of what your parents paid for it, since you owned 1/3 of the property.
I assume that your parents are no longer alive, otherwise you could just ask them. But real estate sales are public record. The information should be available from the local government office that records deeds. A real estate agent or real estate lawyer should be able to find it for you. If the property is a home, first look on Zillow and see if the information is there.