The title King (王 wáng) was the highest noble rank under the emperor. Holders were enfeoffed with a kingdom (王国) and stood above dukes and marquises. In the Three Kingdoms period, the title was used by the dominant figures before they or their successors took the imperial title.
Han precedent
Under the Han, kings were usually members of the imperial family. After the Rebellion of the Seven States (154 BCE), the court stripped kings of most military and fiscal power; they retained prestige and revenue but could not easily challenge the centre. The title remained prestigious and was carefully controlled.
Cao Cao, Liu Bei, and Sun Quan
Cao Cao was enfeoffed as King of Wei (魏王) in 216, formalising his dominance over the Han court. He never became emperor; his son Cao Pi received the abdication and founded the Wei dynasty. Liu Bei declared himself King of Hanzhong (汉中王) in 219 after securing Hanzhong, then emperor of Shu Han in 221. Sun Quan was enfeoffed as King of Wu (吳王) by Wei before breaking with Wei and later declaring himself emperor of Wu. In each case, the title King was a step between warlord or general and sovereign.