Look at your IF:
IF num1 > num2 AND num3 THEN
That doesn't do what you think it does!
You think it means "if number one is the greatest of the three numbers" - it doesn't. The AND breaks it into two separate conditions and insists that both must be true: So "if number one is greater than number two" AND "number three all on it's own".
Try this:
IF num1 > num2 AND num1 > num3 THEN
But why the heck are you learning QBasic in 2016? It's pretty much a dead language, and not something anyone should be trying to learn coding from! It hasn't been distributed since Windows 2000! :laugh:
Look at VB (or better C#) and a (free) copy of Visual Studio Community Edition - it'll make your life a lot, lot easier!