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The Armorer's Cookbook 3 is one of the many Key Items found throughout Elden Ring, sold by the Mistwood Nomadic Merchant in East Limgrave.
The Armorer's Cookbook 3 is a Key Item Recipe Book that will unlock extra crafting recipes once you have obtained it.
This includes the following recipes:
You can purchase the Armorer's Cookbook 3 from the Nomadic Merchant located in the Mistwood area of East Limgrave, just south of the Minor Erdtree along the road. It costs 2,000 Runes.