I've never seen a triple diamond, but I have seen where some of the double diamonds will have and EX inside them, presumably for Extreme.
A double diamond that you can even think about grooming? Absurd. I get that trail ratings are relative, but the tesxt designations I've seen are Easiest, More Difficult, Most Difficult, and Expert Only. So the double diamond designation is a departure from the relative difficulty at that particular hill scale and designates that advanced skills are required to make it down safely. I think Anachronism above gives an excellent description of what element should be present to warrant a double diamond designation. I think further subdividing int triple diamond or EX is superfluous and silly.