expression
three kinds of beneficial friends: straightforward, sincere, and well-informed
A Confucian yojijukugo. Refers to the three types of friends who help one improve: the upright, the sincere, and the knowledgeable. Often contrasted with 損者三友 (three harmful friends).
Antonyms: 損者三友 (そんしゃさんゆう)
益者三友とは、正直な友、誠実な友、博識な友のことだ。
The 'three beneficial friends' are the honest friend, the sincere friend, and the knowledgeable friend.
The antonym: three kinds of harmful friends (hypocritical, flattering, and smooth-talking).
From the Analects of Confucius. The exact historical derivation is uncertain, but the phrase is conventionally associated with Confucian teachings on friendship.