Ha!!! I figured it out. I saw another store here doing it. You create a new listing with the price of the listing set at the BOGO price and let the customer choose whichever ones they want.
For instance, in my shop, only the Ruffle Flower Hair Pins, and Ruffle Flower Barrettes will be on sale. Because their prices for the hair pins are different from the barrettes, I will create a listing for each. The BOGO listing for the hair pins will be set at $14 (doing buy two, get one free), and the barrette BOGO listing will be at $20.
Make sure to add in the description for them to tell you which three (or however many you would do) they want.
Yay!!