I get 3 "BLANK" pop-up CMD boxes - every time that my Win 8.1 laptop is booted up - they appear on the screen, 5 minutes after every start up - they refuse to close by selecting the X in the corner - they close on their own after 3 minutes !!

I have run AVG as well as Malwarebytes - and tdsskiller - nothing detected

What could be the cause of these pop ups ?

Is it some setting that I may have changed or is it something that should concern me ? 

Hi,

Have you set any scheduled task or logon/startup script?

Check your start up items:

Open the Task Manager->Startup tab, check if it's in the list.

I have research some third-party software could cause this issue.

If it's not in your startup list, I would like to suggest you perform an clean boot to see if this issue occur:

How to perform a clean boot in Windows

https://support.microsoft.com/en-us/kb/929135/