Hello, Welcome to the community. 
Images with SD/USB in the name are intended to be burned to and booted from an SD card or USB drive.
Images with EMMC in the name are meant to be flashed over USB, for instance, using the USB Burning Tool.
Both types of images can be found here.
Check that you are using the latest version of the USB Burning Tool, found here?
Also the Khadas in-house utility called Krescue can be used as well to flash images to the VIM3.