Green’s Dictionary of Slang

up a tree n.

[rhy. sl.]

(bingo) the number three.

1981		P. Wright Cockney Dialect and Sl. 109: 3 = you an’ me or up a tree.
1992		R. Puxley Cockney Rabbit.
2002		B. Kirkpatrick Wicked Cockney Rhy. Sl.