The Sony DSC-RX100 III camera has many great features, including a pop-up electronic viewfinder, a great lens with a wide aperture, a tilting LCD screen, and Wi-Fi capability. One feature it lacks is a flash shoe. So, in order to use an external flash unit with this camera, you need to use a flash with optical slave capability. I have put together a short video that demonstrates the use of an external flash with the optical slave feature. Here is the link to the video on YouTube:
For more information about using an optical slave flash with the RX100 III, or for general information about the camera’s features and operations, see Photographer’s Guide to the Sony DSC-RX100 III, available through this site and through various sellers.