Cook potatoes in boiling water in large saucepan 15 min. or until potatoes are tender; drain.
Mash potatoes.
Add sour cream, cream cheese and milk; mash until blended. Spoon into microwaveable serving bowl; cover with plastic wrap. Refrigerate up to 2 days.
Meanwhile, melt butter in large skillet on medium-high heat.
Add mushrooms; cook 5 min. or until mushrooms are tender, stirring frequently. Spoon into bowl; cover. Refrigerate until ready to use.
Vent plastic wrap covering potatoes by poking a few holes in the top just before serving. Microwave on HIGH 6 to 8 min. or until heated through; stir. Top with mushrooms. Microwave, uncovered, 2 to 3 min. or until heated through.