Wikipedia
Triple Tree
The Triple tree was the name referred to the typical structure of Tyburn gallows, composed by a beam placed across the branches of two trees. The triangle shape of this structure provided it the name of the Triple Tree, used the first time in 1571, at the time of the execution of the Catholic cleric John Story.
Usage examples of "triple tree".
But this time there was a new twist at the end of the story: John Cole had asked for the two surviving Shaftoe boys to meet him at the Triple Tree on the morning of his execution.