John Gill Commentary Genesis 36:21

"and Dishon and Ezer and Dishan: these are the chiefs that came of the Horites, the children of Seir in the land of Edom." — Genesis 36:21 (ASV)

And Dishon, and Ezer, and Dishan
These were three others of the sons of Seir, which with the other four before mentioned made seven:

these [are] the dukes of the Horites, the children of Seir in the land
of Edom ;
these were in the land of Edom before it was so called and possessed by the Edomites, and whose posterity afterwards became tributary to them.