Is there such a thing as a ground rule triple?

No, there is no such thing as a “ground rule triple.”  Ground rules shall not conflict with official playing rules (OBR 4.05), and ground rules may not supersede book rules (NFHS 4-1-2).  A manager/coach may not propose any 3-base award as a ground rule, because balls that go out of play or become lodged are already covered by book rules (almost always 2 bases, though the time of enforcement varies by scenario).  The only 3-base award in the rule book is touching a batted ball with detached equipment, which again isn’t a “ground rule” – it’s already in the rule book.

I suppose IF a player touched a batted ball with detached equipment and the ball deflected in to dead ball territory, then that would be a 3 base award…but I that is probably getting into semantics at that point.

Also, even on the detached equipment case, the ball is still live (unlike ground rule doubles), so the batter could try to score at his own risk.  In other words, he could get an inside the park home run anyway.

Tell him no and there’s also no such thing as a ground rule double, either. That will really blow his mind!