What if I bought something that has 3 different parts for a single price but then sold just one of the parts? How do declare the original value of just the sold part?

This is for declaring Sale of Business Property.  If I bought something that has 3 different parts, and each part has a different value, but I bought it for a single price.  And now I've sold just one part of it.  How do I determine the original price I paid for just the single part?