This, much like all the boss encounters in this dungeon, is a simple fight with one catch. As you descend the stairs into the Hall of The Three Kings Alsbeth will summon their ghosts. The first ghost to awaken is Humbart the Bold. Humbart has around 6,600 health and two special attacks worth mentioning. The first is a frontal cleave, so make sure the tank faces him away from any other group members. The second ability is a knockback against his target. To counter this have the tank put his back up against one of the statues in the room.
The Three King's Drops Gloves of the Magus King Laric's Righteous Fists Waistguard of the Bold
After Humbart falls Derribec the Magus will spawn next to the statue on the right as you came down the stairs. Derribec only has 5,200 health and is mostly a push over, only having one ability worth noting. He will use a spell called Life rend on random party members. This can hurt quite a bit but should be easily healed through.
With Derribec down that leaves only one more king, Laric the Ascendant. Laric has 4,900 health and a spell called Unholy Mending that will heal for a significant amount if it is allowed to cast without interruption. Interrupting this heal will speed up the fight greatly but your DPS should be able to burn through Laric easily enough even without the interrupts.
Now that all three have been handled individually, it is time to deal with all three together. The kill order should be Laric the Ascendant because he will spend the entire fight healing either himself or one of the other kings. Then Derribec the Magus to remove his nuke spell from the equation. And finally finish off Humbart the Bold. The best strategy for tanking this part is picking up Laric and Humbart as soon as they spawn and drag them to Derribec. Tank the three of them here with the tanks back against Derribec's statue.