Don’t pick your nose; just pick your friends!
(Ekisha san’yuu, sonsha san’yuu;
“Three beneficial friends, three harmful friends”)
There are three kinds of friends who are good for you and three kinds who are bad for you. The beneficial kinds are the straightforward, the sincere, and the knowledgeable; harmful friends are the ones who are fawning (but dishonest), flattering (but insincere), and smooth-talking (that is, glib but shallow). An admonition to be careful and discerning when choosing your friends and associates; to be wary of people who try to butter you up without actually being good for you.
We begin with the noun 益 (eki), “profit,” “benefit,” compounded with the noun 者 (often mono on its own; in this case sha), “person.” Without any mediating particles, we get the comment on this topic: number-noun 三友 (san’yuu), “three friends.” Then the pattern repeats, but with 益 replaced by the noun 損 (son), “damage,” “loss.”
As you may expect from the all-kanji, no-particle structure, this is another borrowing from Chinese; specifically, our friend the Analects of Confucius (『論語』 = Japanese Rongo).
Incidentally, each half of this phrase may be used as a standalone four-character compound. When used together, the comma is optional, albeit common in modern usage.
(“Daigaku ni hairu mae wa ekisha san’yuu, sonsha san’yuu wo kijun to shite ruumumeeto ya saakuru de kakawaru hitotachi wa ki wo tsukenagara erabou to omotteita kedo, kekkateki ni dou ka na, hanbun gurai seikou shita ka na.”
[“Before going to college I thought that I would choose the people I associated with, like roommates and clubmates and so on, on the basis of the ‘three good friends and three bad friends.’ But I’m not sure how it’s actually turned out; maybe I’ve gotten it about half right?”