Melt butter in a Dutch oven over medium heat; add onions and celery. Sauté 10 minutes or until softened.
Meanwhile, whisk together broth and flour in a small bowl until smooth. Add to onion mixture. Bring to a boil, reduce heat, and simmer 30 minutes or until thickened. Add crawfish; cook 15 minutes. Add lemon juice, salt, cayenne pepper, and hot sauce. Add shrimp, and cook 5 minutes. Add crabmeat, and cook 5 minutes or until heated through. Add green onion and parsley. Serve over hot cooked rice.