The term C3 originated as a way of referring to countries controlled by the game rather than a player. The three Cs are Computer Controlled Country.
In a sentence: I raided a C3 yesterday.
However, this term has alternatively been used to refer to C3s conquered by players, that would technically no longer be C3s, but have not been developed in any way since their conquest aside from maybe some weapons being stuffed in. So in this pejorative sense, C3 refers to a shitty country.
In a sentence: This player uses nothing but C3s to fight.