In a large pot, combine the halved strawberries, raspberries, and blueberries, mashing them slightly with a potato masher to release their juices.
Stir in the lemon juice and let the mixture sit for 15 minutes to soak up the flavors.
After 15 minutes, add the sugar to the fruit mixture, stirring well to dissolve. Let it sit for another 10 minutes.
Place the pot over medium heat and bring the mixture to a boil, stirring constantly. Once it reaches a rolling boil, add the packet of fruit pectin and continue to boil for an additional 1-2 minutes, stirring continuously.
Remove the pot from heat and skim off any foam that has formed on the surface.
Carefully pour the hot jam into sterilized jars, leaving about 1/4 inch of space at the top.
Wipe the rims of the jars clean with a cloth, then seal each jar with a lid and band.
Process the jars in a boiling water bath for 5-10 minutes to ensure they're sealed properly (optional, for longer storage).
Allow the jars to cool completely at room temperature before storing them in the fridge or pantry.
Notes
Serve the jam in decorative jars, and label them with a festive tag. This makes a wonderful homemade gift for the holiday season!