Zach,
I think what you are trying to accomplish is:
Is that accurate?
If so, I would recommend that you import the spreadsheet into an interim entity, lets call it LocationImport, which contains all 3 columns. Then you can build a microflow that will create Locations from this data and match it to Purchases. This approach gives you a lot more control and visibility into the matching process and is the best practice that I follow with these kinds of requirements. If your spreadsheet has a large number of rows, you could set up the microflow to process it in batches and keep track of which objects have been processed already.
Hope that helps,
Mike