Preheat the oven to 375°. Grease a 9 x 13 or similar size baking dish.
Bring a large pot of salted water to boil. Cook pasta according to directions until just al dente. Drain and set aside.
Meanwhile, in a large skillet, cook the bacon over Medium heat until crisp. Remove to a plate, pour off grease, leaving 1 TBSP bacon drippings in the pan.
Add cooked chicken to the pan. Over medium heat, stir chicken until heated thru. Sprinkle ranch seasoning over chicken and stir to coat. Remove chicken from pan.
Over Medium heat, add butter to pan. Once melted, add garlic and cook 30 seconds until fragrant.
Add flour and whisk to form a paste. Slowly add in heavy cream and milk. Whisk to combine. Bring to a low boil and cook 3-5 minutes, stirring occasionally until the liquid is thickened.
Turn heat down to low. Stir in the Parmesan cheese, and half each of the mozzarella and cheddar cheeses until melted. Remove the pan from the heat.
Add chicken and all but 2 TBSP of diced bacon to the cream mixture. Stir to combine. Reserve 2 TBSP diced bacon for garnish.
Pour the cream mixture over cooked pasta and stir gently to coat. Scrape into prepared baking dish.
Evenly sprinkle remaining mozzarella and cheddar cheese over the cream mixture. Dot with reserved bacon.
Bake on center rack of preheated oven for 30 - 35 minutes until cheese is melted and bubbly.
Let pan sit for a few minutes before serving.