Score grapefruit, oranges, and lemons through peel from top to bottom in 6 sections for grapefruit and 4 for oranges and lemons (don't cut into fruit.) Pull off strips of peel with your fingers. Slide a small, sharp knife along inside of peels to remove excess membrane so peels are about 1/4 in. thick. Cut peels lengthwise into strips about 1/2 in. wide in center and tapered on ends.
Put peels in a 3- to 4-qt. saucepan and add cold water to cover. Bring to a boil, then drain. Repeat twice more.
Refill pan with 2 1/2 cups water and 2 1/2 cups sugar; bring to a boil, making sure that sugar dissolves. Add peels and bring to a boil, then reduce heat and simmer gently, stirring occasionally, until peels turn translucent and syrup begins to form bigger bubbles, about 1 1/2 hours.
Drain peels, saving syrup for other uses (such as topping pancakes) if you like. Spread peels on a nonreactive cooling rack set on a parchment-lined baking sheet. Let peels dry overnight.
Put remaining 1 cup sugar in a bowl and toss peels in sugar by the handful, shaking off excess. Put peels on a clean baking sheet and let them dry 1 more day.
Serve peels plain or dipped in chocolate.*
*Melt 10 oz. chopped bittersweet or semisweet chocolate in a small pan over barely simmering water. Dip peels halfway into chocolate; set on baking sheets lined with waxed paper. Chill until set, about 30 minutes.
Make ahead: Up to 3 months, chilled airtight.
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