Boil a large pot of salted water and cook penne pasta according to package instructions until al dente. Drain and set aside.
Season chicken and shrimp with half of the Cajun seasoning.
Heat olive oil in a large skillet over medium-high heat. Brown the sausage slices until caramelized, then remove from pan.
In the same skillet, sear chicken cubes until cooked through and golden, then remove.
Sear shrimp briefly for 1 to 2 minutes per side until pink and opaque, then remove.
Sauté onion, bell pepper, and celery in the remaining fat until softened. Add minced garlic and cook for 60 seconds.
Deglaze the skillet with chicken broth, then stir in crushed tomatoes, remaining Cajun seasoning, and heavy cream. Simmer for 5 minutes until the sauce thickens.
Return the sausage, chicken, and shrimp to the pan. Fold in the cooked pasta and Parmesan cheese, tossing until evenly coated.
Garnish with sliced green onions and serve immediately.