Usually short for
三联书店 (Joint Publishing / Sanlian Bookstore), a major Chinese publisher founded 1948 in Hong Kong from the merger of three left-leaning 1930s houses —
生活书店,
读书出版社,
新知书店 — hence "three joined". Its Beijing flagship store on Meishuguan East Street is a cultural landmark. Also the colloquial name for its flagship magazine
三联生活周刊 (Sanlian Life Weekly), one of China's most respected current-affairs weeklies.