Green’s Dictionary of Slang

third-rate, inadequate, inferior.

1923		Wodehouse Inimitable Jeeves 106: They were a very C3 collection.
1925		(con. WWI) Fraser & Gibbons Soldier and Sailor Words 42: C3: The classification for conscripted men under the Military Service Act of 1916, implying unfitness for active service overseas. The term ‘C3’ has since practically established itself in the language as a term implying inferiority of type.
1930		(con. 1914–18) Brophy & Partridge Songs and Sl. of the British Soldier.
1934		Wodehouse Right Ho, Jeeves 166: The kitchen maid — as C3 a performer as ever wielded a skillet.
1946		‘Josephine Tey’ Miss Pym Disposes 140: ‘I may have a C3 intelligence and childish impulses, but I know tragedy when I see it’.