Rinse the rhubarb and remove any stringy fibers. Cut the rhubarb into small pieces, then place in a pot. Rinse the orange under hot water, then remove the zest (making sure not to remove any of the bitter white pith). Cut the zest into pieces. Squeeze the juice from the orange. Mix the orange zest, orange juice, and the jam sugar with the rhubarb. Set aside and let infuse for 30 minutes.
Rinse and trim the strawberries. Halve or quarter the strawberries depending on their size. Mix the strawberries into the rhubarb mixture. Bring the mixture to a boil, stirring constantly. Cook for 3 minutes, then mix in the citric acid.
Remove any foam that rises to the surface. Fill the prepared screw-top jars to the brim with the hot jam. Seal tightly, and turn upside down briefly to cool. Turn right side up and cool completely.