Let me ask you this:
How did YOU decide which quilt of the three was the "best"?
Judges don't usually know beforehand who the quilt belongs to. They judge by the quality of the work.
Piecing: Do corners/points match? Is the stitching visible? If so, does the thread match? Are there "bra cups"?
Quilting: Are the stitches even, top and bottom? How much quilting is done, is it enough? Are threads/knots buried?
Binding: Is the bindng bias? (It matters to some) Are the corners sharp and properly mitered?
Colors: Are the colors visually appealing? Is the fabric choice appropriate for the pattern? This is the area that has the most "give" and "personal preference" when it comes to judging.
I've never judged a quilt, but was the Needlework Superintendant at the county fair where I lived for a few years, and got to watch the judging. You can go to your local County Extension Office and get a list of guidelines, or you used to be able to. :)