I have a Panasonic DMC-GF3 camera body. I intend to buy a used Olympus Zuiko 35mm f/3.5 ED Macro Lens.
What adapter will I require?
The lens is for a Four-Thirds mount camera. Your Panasonic is a Micro Four-Thirds (MFT) mount camera. Thus, you will need a Four-Thirds to Micro Four-Thirds adapter.