Bring a large pot of water to a boil; add broccoli, cauliflower and carrots. Boil 3 to 4 minutes or until vegetables are crisp-tender. Drain and set aside.
Pour melted butter into a 13"x9" baking dish; add cooked vegetables and set aside. Mix together soup, mayonnaise, cheese and onion in a medium bowl. Spread mixture over vegetables; top with crushed crackers. Bake, uncovered, at 350 degrees for 30 minutes.