There are two ways to boot the VIM3 .. from SD card, from eMMC.
From SD card: write LibreELEC-AMLG12.arm-9.80-devel-20200426060538-38764c8-box.img.gz to an SD card, download Dropbox - u-boot.ext - Simplify your life and place it in the root folder of the SD card, then put the VIM3 board in update mode, it will find the SD card, and LE will install.
From eMMC: follow the SD card install procedure, then run the following commands:
cd /storage
wget https://test.libreelec.tv/LibreELEC-AMLG12.arm-9.80-devel-20200426060538-38764c8-khadas-vim3.img.gz
emmctool w LibreELEC-AMLG12.arm-9.80-devel-20200426060538-38764c8-khadas-vim3.img.gz
This downloads the eMMC install image to /storage and writes it to the internal eMMC storage, overwriting Android. Now power off the board, remove the SD card, and power on again. It should boot into LE on the eMMC. If you ever want to reinstall Android to eMMC, Khadas have an SD card tool called krsecue that will help you do this.
USB/SD Creator works fine for writing SD card images for all the test images (not just KVIM) but install to eMMC is a two stage process.