Grate zest from oranges to equal 1 Tbsp. Repeat with grapefruit. Grate zest from lemon to equal 1 tsp.
Peel and section oranges, grapefruit, and lemon, holding fruit over a bowl to collect juices.
Stir together zest, fruit segments, sugar, ginger, kosher salt, 1/3 cup fruit juices, and 1 3/4 cups water in a large saucepan; bring to a boil. Reduce heat, and simmer, stirring occasionally, 50 minutes or until a candy thermometer registers 225 and mixture is slightly thickened. Cool completely (about 1 hour; mixture will thicken as it cools).
Pour into 2 (8-oz.) jars or airtight containers, and chill 24 hours. Store marmalade in refrigerator up to 3 weeks.
