Meanwhile, melt 2 tablespoons butter in a large skillet over medium-high heat. Add the onion and cook until slightly soft, about 2 minutes. Add the mushrooms and cook, stirring, until they begin to brown, about 2 minutes. Add the remaining 1 tablespoon butter, the chicken, flour, paprika, 1 teaspoon salt and 1/4 teaspoon pepper. Cook, stirring, until the chicken browns, about 3 minutes.