Green’s Dictionary of Slang

triple tree n.

also fatal tree, fatal tripod, gallow(s) tree, gallus tree, sovereign tree, tree, triple trestle, wooden tree
[this giant three-cornered gallows, capable of dispatching 21 villains at a time, stands menacingly in the background of Hogarth’s 1747 engraving of a public hanging]

[16C–1930s] the gallows, orig. that sited at Tyburn, London; thus ext. to other gallows.