Total War: THREE KINGDOMS

You or another faction need to reach King rank before any emperor seats can be created. Dong Zhuo starts off just two ranks below King, so it might be relatively quicker than normal. When you become King, the game will automatically choose the two other strongest factions to also become emperor, so you can try to skew the weighting by occupying or razing the furthest, strongest factions' territories as well as razing the closest faction's capitals. Once the emperor seats are created, simply march your armies to the nearest one and capture it, then choose the raze option from the list.