Bring a large pot of water to a boil. Then, cook the fettuccine to al dente according to package directions. Reserve a cup of the pasta water. Drain the pasta and set it aside.
Melt the butter in a 12-inch cast-iron skillet over medium-high heat. Add sausage and sauté, stirring frequently, until browned, about 5 minutes. Remove it from the pan with a slotted spoon, transfer to a plate, and leave the drippings in the skillet.
Lightly season the shrimp with salt, pepper, and about ½ teaspoon of Creole seasoning. Cook the seasoned shrimp in the same pan, stirring frequently, until just pink, about two minutes. Remove from skillet, and set aside.
Season chicken with 1 teaspoon of Creole seasoning, salt, and pepper. Cook it in the cast iron skillet for 4-5 minutes, stirring a time or two until the chicken is no longer pink inside. Add some more oil to the pan if needed. Transfer to the same plate as the sausage and shrimp.
Add oil to the skillet, then add the onion, bell pepper, celery, garlic, and thyme. Cook, stirring occasionally, until barely soft, about 5 minutes. Add the diced tomatoes and the remaining Creole seasoning. Continue cooking for another 3-4 minutes.
Next, pour in the heavy cream and chicken broth. Stir well. Bring to a boil, reduce heat, and simmer for 5 minutes.
Add cooked pasta and reserved ½ cup pasta water to the simmering sauce. Stir well to coat the pasta with the sauce. Add sausage, shrimp, chicken, and cooking liquid, stir to combine, and cook for about 3 minutes—season with salt and pepper to taste.
Garnish with green onion and lemon if desired. Serve and enjoy.