Preheat oven to 400 degrees F. Heat olive oil in a medium skillet over medium heat. Cook onion, celery and carrot until vegetables are softened, about 5 minutes. Stir in shredded.
chicken, 1/4 cup buffalo sauce, salt and pepper. Set aside to cool slightly.
In a large bowl, stir to combine ricotta cheese, 1/4 cup parmesan cheese, 1/4 cup blue cheese, 1/2 cup mozzarella cheese and egg. Fold in chicken and vegetable mixture until combined.
Lightly coat an 7x10 baking dish with olive oil. Spoon 1/4 cup buffalo sauce into the bottom of the dish. Fill shells with chicken and cheese mixture and arrange in prepared baking dish. Top evenly with 1/2 cup buffalo sauce, 1/4 cup parmesan cheese, 1/4 cup blue cheese and 1/2 cup mozzarella cheese. Bake for 25 minutes.
Top shells with remaining buffalo sauce, a drizzle of ranch dressing and sliced scallions.
To Make the Buffalo Sauce:
Combine all ingredients in a small saucepan over medium high heat. Whisk until butter is completely melted. See aside to cool.