I am really confused on how corpseweft works?
You were correct. When your opponent activated Corpseweft's ability and exiled 3 creature cards from their graveyard, they should have put a single 6/6 black Zombie Horror creature token into play. There is nothing in the text of the ability about putting multiple tokens into play.

In the future, if you have a disagreement like this over how a card or rule works, you should always call a judge. Resolving disputes like this is their primary function.