Slice off the tops and bottoms of the pomelos. Discard tops and bottoms.
Using a peeler, skin two of the pomelos. Peel off, with the yellow skin, as little of the white pith as possible. You should get several long strips.
Julienne the skin strips. Reserve.
Take all three pomelos and slice away as much of the soft white pith as possible from the inner flesh. You will have three balls of flesh the size grapefruits, more or less. Discard the sliced-off pith.
Slice each flesh-ball in half down its central axis.
The collection of fibrous tissue along the central axis of any citrus fruit is called the "rag." Cut out the rag, and any seeds lodged therein, from each of the pomelo halves. Reserve these rags and any seeds. Pomelos usually have small, rather soft seeds.
Grind the resulting chunks of flesh in a food processor. To make sure the pulp is finely ground, strain off the juice and grind again.
Combine ground flesh, juice and julienned skin in a soup pot. Add 3 cups water.
Bring to boil and then simmer, covered, for 20 minutes.
In the meantime, tie the pomelo rags into a sachet using cheesecloth and twine.
Put the sachet into the pot with the pomelos just after you turn off the heat. Make sure the sachet is saturated with the hot liquid.
Refrigerate this mixture overnight.
Next day, squeeze out the sachet into the pomelo mix. Squeeze this out very well as it will provide a lot of pectin. Discard sachet.
Put the pomelo mix into your preserving pan.
Add 6 cups sugar.
Stirring, bring the marmalade to a full rolling boil over high heat.
Cook, uncovered, stirring, and over high heat, until satisfactory gel has been achieved.
The marmalade can be canned. Using appropriate water-bath canning procedures, 6-8 oz jars should be processed for 10 minutes.