If you want to purchase a Class 3 firearm from an estate a class 3 dealer must be used as a transfer agent if the items will be crossing state lines. If the executor, item, and the purchaser are in the same state there is no reason to use a class 3 transfer agent unless your state requires this.
This is the same whether the class 3 firearm is going to an heir of the estate, a business, or an individual.
If you have a question about the transfer of weapons under the National Firearms Act you should contact a Gun Trust Lawyer®.