As far as anyone knows, no one has done a triple hike of each of the Triple Crown of Hiking.
That said ...
None of the three trail conservatories keep very rigorous records; instead they make estimates of trail usage based on trail registers and permit applications. Someone can do the full PCT and the PCTA would never know that they reached the end.
The Triple Crown is more of a popularly conceived achievement than a recognized sporting feat; it was popularized in the early 90s. Only ALDHA-WEST keeps records as to who has completed it, and that pretty much its claim to fame. As far I know, they don't have any records of triples, just doubles. But again, there are Triple Crowners who don't know about or bother with ALDHA.
So it's entirely possible an out-of-the-loop hiker has down three Triple Crowns. But there's no good evidence of that.
The evidence for anyone completing 3 Triple Crowns is scant. If someone has done 3 Triple Crowns, and people know about him or her, that hiker would be famous like Andrew Skurka or Scott Williamson. The fact that this question is being asked just shows that the answer is probably "0".