Baseball, by the Rules
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A Fly Ball Triple
A batter advancing to third base on a base hit, no matter where the ball is hit, and provided there’s no error on the play, is called a "triple". There are some other considerations here, but we’ll get to them in a second.
As you watch examples of this type of play, one thing might stick out— triples on fly balls are almost always the result of the ball landing pretty close to an outfielder, but not quite close enough to be caught. Either the outfielder tries a diving catch and whiffs, or they back up against the wall and leap, but either way, the ball bounces away, giving the batter time to hit third base. Even when the defender has a good chance to catch the ball at the wall, their missing it is almost never called an error, due to the difficulty involved in backing up/jumping with the correct timing— and make no mistake, even if that looks easy to do from your seat, playing the outfield takes skill, especially when no two outfield walls are the same.
Any runners already on base when a triple is hit are pretty much guaranteed to score, unless they screw up. Screw up how? Pretty much the only way would be if they don’t touch one of the bases as they wind their way around. Other than that, they’re scoring, and since we’re talking about a triple, they’re probably scoring without a throw1. If the batter/runner hit a double, and is trying to stretch that double into a triple, then the defense might have a shot an out at third2, but otherwise their only chance for an out is at home.
Of course, just because a batter hits the ball and winds up on third, does not mean they are always credited with a triple. We’ve already talked about errors, but if a batter reaches when there are other runners on base, and while the defense is preoccupied with those runners, the batter/runner moves up to third, that doesn't count as a triple. That's a single or double, with the advance coming on the defensive play. Usually, this means the runner is going to third while the fielders are throwing the ball, so it's called an advance on the throw.