In a spacious pot, combine the halved strawberries, raspberries, and blueberries. Using a potato masher, gently mash the fruits to release their natural juices, creating a thick and vibrant mixture.
Add the lemon juice to the fruit mixture and stir well. Allow it to sit for 15 minutes at room temperature, letting the flavors meld beautifully.
After the resting period, sprinkle the sugar evenly over the fruit and stir continuously until the sugar is fully dissolved. Let it sit again for another 10 minutes, allowing the fruits to absorb the sweetness.
Set the pot over medium heat, stirring constantly and bringing the mixture to a gentle boil. Once it reaches a rolling boil, add the packet of fruit pectin. Keep stirring and boil for an additional 1-2 minutes, ensuring everything is well combined.
Remove the pot from the heat and carefully skim off any foam that may have formed on the surface to ensure a clear jam.
While the jam is still hot, pour it into sterilized jars, making sure to leave about 1/4 inch of space from the top for expansion.
Wipe the rims of each jar with a clean cloth to ensure a proper seal, then securely place a lid and band on each jar.
For longer shelf life, process the jars in a boiling water bath for 5-10 minutes. This step is optional but recommended if you plan to store the jam for an extended period.
Allow the jars to cool completely on a clean kitchen towel or cooling rack at room temperature before storing them in the fridge or pantry.
Notes
Serve in decorative jars with ribbons for a charming gift.