Melt 2 tablespoons of the butter in a large skillet. Add the shallots and cook over low heat, stirring, until softened, about 5 minutes. Add all of the mushrooms, season with salt and pepper, cover and cook until they have released their liquid, about 4 minutes. Uncover and cook over moderate heat until the mushrooms are browned on the bottom, about 3 minutes. Stir and cook for 4 minutes longer. Add the remaining 1 tablespoon of butter and cook, stirring, for 1 minute. Season with salt and pepper and transfer to a saucepan.