In a large saucepan; add 1 in. water. Add cauliflower and carrots. Bring to a boil. Reduce heat; cover and simmer for 4-8 minutes. Add broccoli; cook 4-6 minutes longer or until all the vegetables are crisp-tender. Drain.
In a small bowl, combine the mayonnaise, onion, horseradish, salt and pepper. Add vegetables; toss to coat.
Pour into a greased 2-qt. baking dish. In a small bowl, combine the bread crumbs, butter and paprika until crumbly; sprinkle over vegetables. Bake, uncovered, at 350° for 25-30 minutes or until heated through.