The Triple Crown consists of the Big Three National Scenic Trails: The Appalachian Trail, the Pacific Crest Trail and the Continental Divide Trail. Completing the Triple Crown requires about 7,900 miles of hiking through 22 US States. As of the end of 2018, about 400 people are acknowledged to have completed the Triple Crown. The Calendar Triple Crown (all three in one calendar year) was first done by Flyin' Brian Robinson, in about 300 days between January 1 and October 29, 2001, a feat which has been repeated only five times (as of November 2018). The trails can be hiked in any order, and can be broken into segments, as long as the full length of each trail is covered.
Note on "style": The Calendar Triple has always been done in typical thru-hiker style, which we term "self-supported". However, on such a long hike, most people will receive some support by family and friends from time to time; we generally consider these trips to have still been "self-supported". If the support is planned and meaningful, the hike will be considered "supported".
Note on distances: The current AT mile mileage is listed as 2,189, the PCT 2,655, and the CDT is estimated at 3,100, for a total of 7,944 miles. The AT and PCT have clearly designated routes which must be followed for an FKT; however, the official governing body's can change the route at any time, which then becomes the new official route and distance. The CDT has never had a clearly designated route, which is why FKT's have not been considered meaningful on that Trail.