OK, as it applies to Khadas SBCs, Etcher and similar are used for images designed for booting/running from an SD card or USB attached storage. Etcher and similar will not work for the VIM3L Android images as those are designed to be flashed using the USB Burning Tool or Linux burning tools. This also applies to superceleron’s SC_VIM3L_ATV image(s).
In some cases, the provided images can be used with the Burn Card Maker, though I have not tried it on the VIM3L, the VIM3 and VIM3L are very similar. A Linux method(not tried by me) to make the card is here.
You should be able to enter upgrade mode by using the keys method or TST.
MASKROM mode will be indicated by the VIM’s LED blinking blue for a few seconds and then it will go off.
Let me know what happens when you follow the above guides and instructions. Please let me know at what step it fails.