They depend on a combination of two factors, which are themselves uncommon: In baseball scorekeeping, the abbreviation GITP can be used if the batter grounded into a triple play.
The most recent MLB unassisted triple play is consistent with the above – it occurred on August 23, 2009, by second baseman Eric Bruntlett of the Philadelphia Phillies, in a game against the New York Mets.
[15] Political columnist and baseball enthusiast George Will posed one hypothetical way that a triple play could occur with no fielder touching the ball.
[16] Whenever a batter or runner is out without a fielder touching the ball, MLB rule book section 10.09 provides for automatic putouts to be assigned by the official scorer.
Under the scenario described above, the same fielder (the shortstop, for example) could be credited with all three putouts, thus attaining an unassisted triple play without having touched the ball.
Source:[1] On June 27, 1967, the New York Mets and Pittsburgh Pirates staged a triple play before their game at Shea Stadium for the film The Odd Couple.