Wash 8 half-pint or 4 pint jars. Keep hot until needed. Prepare lids as manufacturer directs.
Discard stems and crush enough strawberries to measure 2 cups of crushed berries. Crush enough raspberries to measure 1 cup. Crush enough blackberries to measure 1 cup.
Add the prepared fruit to a 6- or 8-quart saucepan. Stir in the sugar and butter or margarine. Stirring constantly, bring the mixture to a full rolling boil (a boil that doesn't stop bubbling when stirred) on high heat.
Stir in pectin quickly. Return the mixture to a full rolling boil and boil exactly 1 minute, stirring constantly. Remove from heat. Skim off any foam with a metal spoon.
Ladle the jam into one jar at a time, leaving 1/4-inch headspace. Wipe jar rim with a clean, damp cloth. Attach lid. Fill and close remaining jars. Process in a boiling water canner for 10 minutes (15 minutes at 1,000 to 6,000 feet; 20 minutes above 6,000 feet).