Set the cheddar, mozzarella, and cream cheese out ahead of time to bring them near room temperature.
Place chicken in a pot of water and bring to a gentle boil. Cook for 15 minutes, until cooked through. Remove and shred the chicken.
Toss the shredded chicken with blue cheese dressing and set aside.
Melt butter in a large high-walled skillet over medium heat. Add the onions and celery. Cook for 5 minutes, until softened. Add the garlic and cook for 1 more minute.
Add the undrained diced tomatoes and chicken broth. Bring to a boil and add the pasta. Submerge the pasta into the liquid and let it come back up to a boil.
Cover and let it boil for 10-13 minutes, or until al dente. Stir occasionally to prevent sticking.
Reduce heat to low. Add the buffalo sauce, cubed cream cheese, and shredded cheese. Stir gently.
Add the shredded chicken and red pepper flakes.
Optional: Swirl in 2 Tablespoons COLD butter for a smooth finish.