Preheat oven to 350°F.
Bring a large pot of water to a boil. Add 1 pound elbow macaroni and cook until al dente, about 8 minutes. Drain the macaroni and set it aside.
While the pasta cooks, pour the jar of pimentos (4 ounces) into a strainer, drain well, and chop if not already chopped. Set aside.
In a large pot over medium heat, melt 3 tablespoons unsalted butter. Stir in 3 tablespoons all-purpose flour and cook, stirring constantly, for 1 minute.
Gradually whisk in 2 cups whole milk until smooth. Cook, stirring frequently, until the mixture thickens and begins to bubble. Reduce heat to low.
Add 1 cup shredded sharp cheddar, 1 cup shredded Monterey Jack, and 4 ounces softened cream cheese to the pot. Stir until all cheeses are fully melted and the sauce is smooth.
Stir the drained, chopped pimentos into the cheese sauce along with 1/2 teaspoon salt and 1/4 teaspoon black pepper, mixing until evenly distributed.
Add the drained macaroni to the cheese sauce and stir to coat evenly. Transfer the macaroni and cheese into a baking dish.
Bake uncovered for 25 minutes, or until the top is bubbly and golden. Remove from oven and let rest a few minutes before serving.