A triple stretch in football is a pass route in which three wide receivers run stretch or vertical routes. It is a variation of the hail mary route (when all eligible receivers run vertical patterns), and the difference is with the triple stretch, only three players are running straight downfield. The other eligible receivers may run an underneath pattern.
On top of the potential big gain, the clearing out of the receivers allows for a pass to be completed underneath. Either a tight end or running back would slip out to the recently opened up space. The defensive backs would all be far down field covering the wide receivers, leaving tons of space for the offense to take advantage of.
A triple stretch is primarily used in situations where an offense needs to gain yards and put points on the board quickly. In this type of situation it is pretty obvious what the offense is trying to do. A good offense will actually run this play in scenarios where the defense does not expect it to be run. If the defense is keyed in as to what the offense is planning on doing, it has a low probability of working (see below).
First off, the triple stretch has a low probability of resulting in a long catch. That being said the correct defensive scheme still needs to be called. Defenses should play man to man defense with each cornerback giving a large cushion to the receiver. The most important part though is what the safeties are doing. The safeties need to make sure the receivers stay in front of them, that way they have a better chance of knocking the ball down or even intercepting it.