Maybe god is out on vacation 
If you have an SD card try using that with Ubuntu focal 20.04,
I am using that for the past couple of days and I have had no problem like that.
could you try Ctrl + Alt + F7 or Ctrl + Alt + F1 and see if something happens ?
the strange problem is very, maybe the screen resolution problem for Ubuntu is in your case. Do you have another HDMI cable?
What are you using to power your VIM3? A 5 volt power supply at 2 amps or better is recommended. Even better, the Khadas 24W PD supply, found on Khadas’ shop and Amazon.
Do you have a USB-to-Serial adapter tool(Serial tool)?