Place potatoes in a 6-qt saucepan; add enough water to cover potatoes. Heat to boiling over high heat; reduce heat to medium. Cook uncovered 15 to 18 minutes or until tender; drain. Mash potatoes in saucepan with potato masher or elecric mixer on low speed.
Meanwhile, in medium bowl, beat cream cheese, cheddar cheese and parmesan cheese with elecric mixer on low speed until smooth. Beat in sour cream and garlic salt.
Heat oven to 350. Stir cheese mixture into mashed potatoes until well blended. If potatoes are too still, stir in milk, 1 tablespoon at a time, until desired consistency. Spoon into ungreased 13X9 inch glass baking dish.
(If not baking right away, cover and refrigerate. Remove from refrigerator 30 minutes before ready to bake and then bake as directed.).
Bake uncovered 35 to 40 minutes or until hot and top is lightly browned. Sprinkle with paprika and chives.