After your quilt is quilted, it has several inches of extra batt and backing on all sides, correct? If the top is square, that is the edge you use to sew on your binding. No matter how wide the binding is, that is the edge you use. That way you won't cut off points. I will trim a little of the batt and back at this point just to get excess out of my way. Leaving about an inch would work fine.
After the binding is sewn on, the batt and back need to be trimmed. The measurement starts at the sewing line you used to sew on the binding. With a 2 1/2 inch binding I usually cut at 3/8 of an inch. That leaves just enough to fill the binding. I you made a 3 inch binding, you might need 3/4 inch from the sewing line. You will have to try it out, and don't forget to allow for the thickness of the batt.