Prepare: Sterilize 9 8 oz jam/jelly jars and prepare two-piece canning lids according to manufacturer's directions (this may be one more than you need, but it's nice to have in case of overflow jam).
Sort and wash fully ripe strawberries; remove stems and caps. Lightly crush strawberries, you want to have some solid chunks of fruit. Crush raspberries and blackberries fully.
Measure out sugar into a bowl, set aside. You will be adding sugar all at once.
Cut top off liquid pectin pouch, set pouch upright in a glass for easy access.
Cook: Add berries and lemon juice to pot, bring to simmer. Add sugar and stir well. Place on high heat and, stirring constantly, bring quickly to a full boil with bubbles over the entire surface that cannot be dissipated by stirring. Boil hard for 1 minute, stirring constantly. Stir in pectin. Return to boil and cook for one minute more. Remove from heat, skim foam.
Jar: Funnel hot jam immediately into hot, sterile jars, leaving ¼ inch headspace. Wipe rims of jars with a dampened clean paper towel; adjust two-piece metal canning lids.
Process: Place completed jars in a Boiling Water Canner on full boil for 10 minutes (or recommendations based on your altitude). Remove from water, let sit for 5 minutes before removing jars to towel lined counter/table to set for a minimum of 24 hours.