Mounted an old (gift) Leupold VX lll (3.5-10x) scope on my AR-15. Started sighting in at 25 yard line to get shots on paper.

Image was extremely fuzzy at 25 yards (almost a "double image" for the 1" orange dot I was using as a target) and was still pretty fuzzy at 50 yards. Image was clear at 100 yards.

Is this common ??   What's the minimum range at which a good (Leupold, Burris) scope should focus ??

does it have and adjustable focus?
what is the parallax set at on the scope? usually 75-100yds on rifle scopes.

As a general rule, most scopes for a centerfire rifle without an adjustable objective have a fixed focus at 100 yards. So, to have a fuzzy image of an object at 25 yards is normal. If the image was blurry to a point of being indistinguishable, then there's a definite problem.

What fixed focus means is that parallax error has been reduced to its minimum at that distance. 100 yards is normally chosen because that gives you a range of (from most manufacturers) 50 to 150 yards where parallax error shouldn't be a problem for normal shooting.

If you need more accuracy, then you would have to use a scope with an adjustable objective to remove parallax error at a specific distance.