Melt 3 Tbsp. butter in medium saucepan on medium heat. Whisk in flour; cook 2 min., stirring constantly. Gradually stir in milk. Bring to boil; cook and stir 3 to 5 min. or until thickened. Add Velveeta
; cook 3 min. or until melted, stirring frequently. Stir in macaroni.