Green’s Dictionary of Slang

earth n.1

also eerht, eerth, erth
[backsl.]

three; thus earth/erth sis noms, a three-month prison term.

[UK]Hotten Dict. of Modern Sl. etc. 124: erth, three.
[UK](con. 1840s–50s) H. Mayhew London Labour and London Poor I 411/2: There’s people there talk backward – for one they say eno [...] for three eerht.
[UK]Sl. Dict.
[UK]Edinburgh Eve. News 26 Dec. 3/5: ‘Eno’ is one; ‘owt,’ two; ‘eerth,’ three; ‘rouaf,’ four; ‘exif,’ five; and ‘xis’ pronounced exes, six.
[UK]Lloyd's Wkly Newspaper 3 Jan. 6/5: 'Here's erth-yeneps [...] and I wish it were erth-gens.' 'Never mid, Mick [...] It's better than erth-sith-noms'.
[UK]J. Franklyn Cockney 297: Erth (three).