Wash fruit. Cut grapefruit into 8 wedges and orange and lemon into 4 wedges. Then cut wedges crosswise into thin slices; save juice. In 6 quart kettle, combine all fruit, fruit juice and 1 1/2 quarts water. Bring to boiling; boil rapidly, uncovered for 30 minutes or until peel is tender. Measure fruit mixture; return to kettle. Add 1 cup sugar for each cup of fruit mixture. Bring to boiling, stirring, until sugar is dissolved; boil rapidly, stirring frequently 30 minutes or until mixture is thick and reaches jellying point. Meanwhile sterilize 8 jelly glasses by boiling them 20 minutes. Keep in hot water until ready to fill. Lift out with tongs. Remove marmalade from heat. Ladle immediately into hot, sterilized glasses, filling to within 1/2-inch of top. Cover immediately with 1/8-inch of hot paraffin. Let glasses stand overnight or until cool. Cover glasses with metal lids. Makes 7 or 8 glasses. *To test for jellying point: dip a large metal spoon into boiling syrup; tilt spoon so syrup runs off edge. Syrup is at jellying point when it doesn't flow, but divides into 2 drops that run together and sheet from the spoon.