Tuya-Convert might not be possible for this device since the template was added (2019-12-04). All new Tuya devices have firmware preventing flash but it is still possible to Tuya-Convert old stock devices without connecting them to the Tuya app.
The U3S appears to be a clone of the BlitzWolf SHP2.
Box of 4 ordered from Amazon UK 23/08/20 flashed with TuyaConvert successfully. Individual socket ordered from Amazon 30/11/20 flash with TuyaConvert successfully.
Box of 4 ordered from Amazon.co.uk 06/12/20 cannot be flashed with TuyaConvert.