Watching those YouTube videos seems like you may have a bad Riser or Riser cable or even the Motherboard's slot where the 3rd Riser is connected is defective.
Since you mentioned you tried all 3 GPUs in combinations of twos and they all worked correctly. That indicates the 3rd Riser, cable, or Motherboard slot where the Riser is connected may be defective.
What does Device Manager show when you have all 3 GPUs connected? is it recognized or error code 43 or any other type of error or yellow exclamation points?
What Mining program and OS are you using (Windows or Linux)?