did you follow this instructions on Coreelec Website? First ensure u have android on emmc so the uboot is legacy. Best way to ensure this is to use USB Burn Tool and flash khadas official android image to your vim3. You can get that image here:
https://dl.khadas.com/Firmware/VIM3/Android/?C=M;O=D
Latest Burn Tool can be found here:
https://dl.khadas.com/Tools/USB_Burning_Tool_v2.2.4.zip
Then download the coreelec image and flash it to sd card… All the steps are on coreelec website:
**New installation**
Prepare an SD card *(class 10)* / USB flash drive as follows;
***NOTE**: On some devices, only 1 USB port is able to boot the device, you may have to try them all to see which one works for you, some boxes are also unable to boot from a USB drive.*
1. Download the latest ***.img.gz*** file from [here ](https://github.com/CoreELEC/CoreELEC/releases/latest).
2. Use [Rufus ](https://rufus.ie/), [usbimager ](https://gitlab.com/bztsrc/usbimager/) (or your preferred image writing software) to burn the downloaded image file to your SD card / USB flash drive.
3. If successful, the SD card / USB flash drive capacity will be shown as 512MB, this is OK as there is a second (hidden) partition for CoreELEC user data.
4. Browse to the ***Device Trees*** folder, now present on the SD card or USB flash drive, and copy the correct [***.dtb*** ](https://discourse.coreelec.org/t/which-dtb-do-i-use/389) image file for your device to the root folder of the SD card / USB flash drive, and rename it to ***dtb.img***.
*To boot the system **for the first time** you have to use one from the following methods, sometimes only one of these methods will work!*
* ***Khadas VIM***: insert the CoreELEC prepared SD card and boot into Upgrade Mode, CoreELEC will start automatically: [ Howto Boot Into Upgrade Mode - Khadas ](http://forum.khadas.com/t/boot-images-from-external-medias-like-sd-card-and-u-disk/1948)
Another option is to use KRESCUE to flash officially avaialble Coreelec nighly to vim3. It can be found at below link, but u just boot vim with a krescue sdcard and rest can be done inside its wizard.
https://dl.khadas.com/Firmware/VIM3/Coreelec/VIM3x.COREELEC.9.2.5.emmc.kresq (DONT DOWNLOAD IT, KRESCUE WILL DOWNLOAD IT FOR YOU)