The triple jump is an obstacle that most people add to their course later on, but more often than not, it is an obstacle that many dogs stumble over in an agility trial! The challenge is for your dog to clear the jump by not only jumping high, but *long*. This is also why it is sometimes called a "Spread Jump" (except technically a spread jump is a little more lenient in the placement of the bars, but the principle is the same).
What you see in the picture is a Triple Jump set at the height of 16". The back bar is 16", the center bar is set 8" apart (and a little lower), and the bottom bar another 8" apart (and again a little lower). So basically, the dog must jump 16" high, AND 16" wide. The rule is to space your bars half the distance of your dog's jump height. So if your dog jumps 24", you space your bars 12" apart, AND in a descending order.
This practice jump is fully adjustable. The 2 vertical bars you see inside the square side-frames *slide* back and forth horizontally so you can space your bars apart as wide or as narrow as they need to be. The bars rest on 3 pairs of clip-on jump cups that also slide, up and down on the vertical poles.