Place tomatoes in a very large non-reactive saucepan.
Halve and seed oranges and lemon. Finely chop fruit in food processor or blender and add to tomatoes. Bring mixture to a full boil over high heat. Slowly add sugar, stirring until sugar is completely dissolved. Return to a boil and boil rapidly until mixture forms a gel, about 1 hour, stirring frequently. Remove from heat.
Ladle into hot jars and process for 10 minutes in a boiling water bath.