Bring a large pot of salted water to a boil, then add the pasta shells. Cook according to package instructions until al dente, then drain and set aside to cool slightly.
In a sauté pan, melt the butter over medium heat and add the minced garlic. Sauté for about 1 minute until fragrant, with a gentle sizzling sound, but not browned.
Add the cooked chicken to the garlic and stir to combine, cooking for another 2-3 minutes to warm everything through. Season with salt and pepper to taste.
In a mixing bowl, combine the Alfredo sauce with the sautéed chicken and garlic mixture. Mix until evenly incorporated.
Carefully stuff each cooked shell with the chicken and Alfredo filling using a small spoon or piping bag for neatness, then place them seam-side down in a greased baking dish.
Pour the remaining Alfredo mixture over the filled shells, ensuring they are mostly covered with sauce and placed close together.
Sprinkle shredded mozzarella, Parmesan, and cheddar cheeses evenly over the top of the shells and sauce, creating a cheesy crust that will melt and brown.
Preheat your oven to 375°F (190°C) and bake the assembled dish uncovered for about 25-30 minutes, or until the cheese is golden and bubbly.
Once baked, remove the dish from the oven and let it rest for a few minutes to set. The edges will be crispy, and the cheese on top will be lightly browned.
Serve hot, with an extra sprinkle of Parmesan or fresh herbs if desired, and enjoy the creamy, cheesy stuffed shells with their bubbling topping.
