三一三十一

phrase

an abacus mnemonic for dividing by three; (figurative) to split equally into three parts; to go Dutch three ways

Originally describes the abacus calculation step where 3 goes into 10 three times with a remainder of 1 (hence the numbers 3-1-3-10-1). In modern colloquial usage, it is commonly used to mean splitting a bill three ways, with each person paying one-third.