Using a citrus reamer (you get far more out that way) squeeze the juice of the grapefruits and lemon into a bowl, getting rid of the seeds. Remove the membranes and set the seeds and membranes into a square of cheesecloth, bundle it up and tie with a string. Place in a large, heavy saucepan with the juice.
Stack the grapefruit skins on a cutting board (peel away any excess white pith with your fingers first) and cut into thin strips. Cut crosswise so that the pieces are no longer than an inch. Add them to the pan.
Add 5 cups water and bring to a simmer over medium heat. Reduce heat, cover and simmer, stirring often, for about 2 1/2 hours or until the peel is very soft. Remove the bag and let cool. You should have about 5 cups. Add the sugar and rhubarb, bring to a boil and cook, stirring often, for about 20 minutes, or until the mixture gels when dropped onto a chilled plate; it should wrinkle when pushed with your finger. Continue to cook until it reaches that stage. If it doesn't, add a package of pectin and boil hard for a minute or two.
Pour into clean, hot jars, leaving 1/4-inch headspace. Seal and boil in a boiling water bath for 10 minutes for longer-term storage, otherwise store in the fridge or freezer.