Combine rhubarb and water in a pot. Bring to a boil over high heat. Cover, reduce heat and simmer 5 minutes. Stir frequently. Add blueberries and raspberries, lemon juice and Certo. Mix to combine. Add butter to reduce foaming. Return to a boil. Add sugar, stir and return to a full rolling boil. Stir constantly and boil hard for one minute. Remove jam from heat and stir; skim off the foam from the top. Ladle into hot, sterilized jars, leaving a ¼-inch space at the top. Clean rims of jars, seal with lids. Store in a cool, dark place.