Tuya-Convert might not be possible for this device if the manufacturer started using newest Tuya firmware since this template was published (2019-12-04).
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.