Preheat oven to 350º.
Place cauliflower and carrots in large saucepan with small amount of water, cook over medium heat for about 3 minutes; add broccoli and continue to cook for about 6 more minutes, or, until all are crisp-tender, remove from heat and set aside. Combine the mayonaise, onion, horseradish, salt & pepper, add to vegetable and mix thoroughly. Pour into a greased 2 quart casserole dish. Combine the bread crumbs, melted butter and paprika...sprinkle over the casserole.
Bake for 25-30 minutes.