Who is the Third?
Although Lucifer Morningstar is always the Head of the Triumvirate and Beelzebub is always one of the other two lords, the third member has been two different demons. Originally, in The Sandman #4 where the three first appeared, it was Azazel. However, it was changed to better fit a tale in The Demon where Etrigan's father, Belial, was now seen as the third. Doug Wheeler's Swamp Thing explained this change by revealing that since Hell is made up of belief and most believe Lucifer is the sole ruler, the other Lords have to use most of their power just to keep their position and Azazel didn't make the cut.
When Etrigan overthrew Lucifer, Beelzebub, and his father Belial, Etrigan took Lucifer's position as the King of Hell, he gave Beelzebub's position to the Guardian of the Pit, Abadon, and turned his father Belial's position in Hell to his mother.