Preheat oven to 350°. In a small bowl, mix mayonnaise and mustard. Spread bun bottoms and tops with mayonnaise mixture. Layer bottoms with ham, chicken and cheese; replace tops. Arrange in a single layer in a 15x10x1-in. baking pan.
Bake, covered, until sandwiches are heated through and cheese is melted, 10-15 minutes .