Prepare the citrus peels. Using a vegetable peeler, remove the outer peel from the oranges, grapefruit, and lemon, avoiding as much white pith as possible. A little pith is fine, but keep most of it on the fruit. Roughly chop the peels and place them in a small saucepan with 3 cups of water.

Simmer the peels. Bring the peels to a gentle simmer and cook for about 30 minutes, until softened and aromatic. Remove from the heat.

Make a pectin bundle (optional but recommended). While the peels simmer, cut away the remaining white pith from the citrus fruits. Separate the fruit into segments, removing any tough membranes and seeds as you go. Gather the discarded pith, membranes, and seeds and tie them loosely in a piece of cheesecloth to form a small bundle. This bundle releases natural pectin, helping the marmalade set.

Prepare the fruit. Roughly chop the citrus segments and add them to a large, heavy-bottomed pot. Mash lightly with a potato masher or your hands to release some juice.

Combine the marmalade base. Add the sugar, honey, chopped peels with the liquid they cooked in, and the pectin bundle (if using) to the pot with the fruit. 

Cook the marmalade. Bring the mixture to a steady simmer over medium-low heat. Give it a stir every once in a while. Cook for about 25 minutes, then remove the pectin bundle.

Finish. Continue cooking for about 10 more minutes, or until the marmalade has thickened, is darker in color, and no longer appears watery.

Store. Spoon the hot marmalade into clean jars. Let cool completely, then cover and refrigerate. The marmalade will continue to thicken as it cools and will keep in the fridge for several weeks.