狐狸精 is not fox, in Chinese means the women who know how to seduce men. It is usually being the pronoun of bad woman, mistress and slut. For example: 这女人是个狐狸精，抢了闺蜜的老公。(This woman is a slut who seduced and stole her best girl friend's husband.)
小三 doesn't mean small 3, the true meaning is mistress, she's the third person out of someone's marriage. For example: 她是老板的小三。（She's her boss's mistress.)
小白脸 doesn't mean a small white face, it means a young male mistress. For example: 那个老女人养了个小白脸。（That old woman has a young male mistress.)