Male equivalent of 小三?
A proper term would be 情夫:


  男女两人,一方或双方已有配偶,他们之间发生性爱的行为,男方是女方的情夫


小三 can also be applied to a male as it means the third party in an affair or 第三者.

二公 and 二爺 are informal terms opposite to 二奶.

小白脸 does not necessarily refers to a man who has committed adultery with a married woman. It can also mean the the guy is living off the woman or that the guy has a fair complexion.