Step 1Using a sharp vegetable peeler or paring knife, cut the thin outer layer of pomelo peel (without any white pith) into 1- to 2-inch wide strips. Stack the strips and cut crosswise into very thin slices; you should have about 3/4 cup. Combine the peel and enough cold water to cover by 1 inch in a heavy medium saucepan and bring to a full boil over medium-high heat. Remove the peel from heat and pour off all of the water. (Reserve the peel in the saucepan.)
Step 2Supreme the pomelos: Using a sharp knife, cut off the top and the bottom of the fruit so it will sit flat on the cutting board. Starting where you see the pomelo separate from the white pith, cut away one section of pith, following the line of the fruit. This will expose the underlying fruit. Continue cutting away sections of the pith until only fruit remains. When you're done, go back over the fruit, removing any traces of pith. Working over a bowl, separate each segment; you should have about 1 1/2 cups pulp. Strain the pomelos, reserving the juice. Peel off the pith and separate the pulp from the membrane; discard the pith and membrane. (You should have about 1 1/2 cups pulp.) Strain the pulp, reserving the juice. Add enough additional pomelo, orange or grapefruit juice to the strained juice to measure 1 cup.
Step 3Add the juice, along with the pulp and cardamom pods to the peel. Bring the mixture to a simmer over medium heat. Continue cooking, stirring frequently, until the pulp separates into small pieces and the mixture is reduced to 1 3/4 cups, about 20 minutes. Stir in the sugar and continue cooking, stirring frequently until the marmalade thickens slightly and is reduced to 2 1/4 cups, 10 to 15 minutes. Transfer the marmalade into jars and cool completely. Seal the jars and refrigerate up to 2 months or can following canning instructions.