Three Ones Hotel, n. — Green’s Dictionary of Slang

Three Ones Hotel n.

[a popular meeting place for Australians in London at the time. Downing, Digger Dialects(1919), suggests ‘i.e. “one arm, one eye and one pedestal”’, but the un-bowdlerized appendage is ‘one arsehole’]

(Aus.) Nelson’s Column, Trafalgar Square, London.

1919		W.H. Downing Digger Dialects 49: three i’s hotel — The Nelson Monument, in Trafalgar Square, London.
1924		(con. WWI) A.G. Pretty Gloss. Sl. [...] in the A.I.F. 1921–1924 (rev. t/s) n.p.: the three ones. A reference to the statue of Nelson in Trafalgar Square. The Square was known to Australians as a favorite meeting place; and called the Three Ones Hotel. The ‘ones’ are Nelson’s one arm, one eye and one posterior orifice.
[1978		A. Burgess 1985 (1980) 208: Trafalgar Square [...] high in air, the one-one-eyed, one-armed (and, as the vulgar had it, one arsed) hero].