Remove rind from grapefruit; set aside. Chop pulp, and discard seeds. Set pulp aside.
Cut reserved rind into small star-shaped pieces using an aspic cutter; reserve remaining rind pieces. Combine grapefruit rind stars, reserved rind pieces, orange rind, and 1 1/2 quarts water in a large Dutch oven; bring mixture to a boil. Boil, uncovered, 5 minutes; drain. Repeat procedure.
Combine remaining 1 1/2 quarts water, rind, chopped orange, and lemon slices in Dutch oven; bring to a boil, and boil 5 minutes. Cover and let stand 12 to 18 hours in a cool place.
Uncover; bring to a boil. Reduce heat; cook 35 minutes or until rind is tender. Measure amount of fruit and liquid mixture. Add 1 cup sugar per 1 cup mixture; stir well. Bring to a boil; cook, stirring frequently, until mixture registers 220° on candy thermometer.
Pour marmalade into hot sterilized jars, leaving 1/4-inch head space; cover at once with metal lids, and screw bands tight. Process in a boiling-water bath 10 minutes.