1. Taste the blood orange juice. Add 2 tbsp. sugar and 1 tsp. lemon juice and stir until sugar is completely dissolved. Taste again to see if it has the right sweetness and tartness for you, bearing in mind that, when frozen, the juice will taste more subdued; add up to 1 tbsp. sugar and 1 tsp. lemon juice if needed. Repeat with tangerine and grapefruit juices, using the same range of sugar and lemon juice for each.
2. Fill each of 8 popsicle molds (see Notes) 1/3 full with blood orange juice and freeze, making sure they're level and upright, until firm to the touch, about 45 minutes.
3. Fill each mold 1/3 more with tangerine juice and freeze just until firm to the touch, another 45 minutes. Carefully insert sticks, leaving 1 1/2 to 2 inches of each sticking out. Freeze until sticks feel solidly anchored, about 45 minutes.
4. Fill each mold to top with grapefruit juice, cover, and freeze 2 hours or overnight.
5. To unmold, run warm water over the individual molds just until popsicles release from sides, 5 to 15 seconds.
Note: Nutritional analysis is per popsicle.