The Underking is Wulfharth, Arctus, and Talos. He is all three at the same time, while being separate. The Underking is equal to 111. Talos equals 1, Wulfharth equals 1, and Arctuc equal 1. Together they are indistinguishable, apart they are unique.
You can't think of the Underking as a person. You should rather see the name as a title, the one who claims the rebel side of the Enantiomorph.