Place the walnuts, pecans and Brazil nuts in a single layer on a baking sheet. Bake at 350° for 4-8 minutes or until toasted and golden brown, stirring once. Cool on a wire rack. Line an 8-in. square pan with foil; grease the foil with butter and set aside.
In a large heavy saucepan, combine the sugar, cream and corn syrup. Bring to a boil over medium heat, stirring constantly. Stir in the toasted nuts. Cook, without stirring, until a candy thermometer reads 238° (soft-ball stage).
Remove from the heat. Stir with a wooden spoon until creamy and thickened. Quickly spread into prepared pan; cool. Cover and refrigerate for 8 hours or overnight.
Using foil, lift candy out of pan; discard foil. Cut candy into squares. Store in an airtight container in the refrigerator.