Place several small plates or flat bowls in the freezer for the gel test after the mixture cooks.

Prepare the strawberries by washing the berries and cutting off the tops. I also cut very large berries in half. Wash the raspberries and blueberries. Discard any mushy/bad fruit. If any leafy tops on on the raspberries, remove those as well. Juice the lemon if you are using fresh lemon.

Place the raspberries and sugar in a medium-sized saucepan and cook over medium heat until the sugar has melted. This should take about five minutes. Remove from heat and run the cooked raspberries through a sieve or mesh of some sort. Press and mash the raspberries over a bowl to remove the seeds, preserving as much pulp as possible. This might take a few minutes. Discard the seeds.

Return the deseeded raspberry sugar mixture to the pan. Place the prepared strawberries, blueberries, lemon juice, and salt in the saucepan with the deseeded raspberry mixture and cook over medium heat for five minutes. It can gently boil/simmer but do not allow it to come to a hard boil.

Turn off the heat and mash the berries until they are the size you'd like. The trickiest part will be to mash the blueberries depending on how large your potato masher holes are. This will take a couple of minutes.

Return the mixture to a boil, then turn the heat down and simmer for about 20-30 minutes without stirring. If you are using a candy thermometer, cook until it reads 210℉ (see notes). AND do the gel test - see the next step.

To do the gel test (even if you use a candy thermometer), do your first test at about 20 minutes. Turn off the stove while you complete the gel test. Spoon a small amount (one-half teaspoon) onto the cold plate. Return to the freezer for two minutes. Tilt the plate to see if the mixture runs down the plate quickly or if it very slowly creeps. If it runs quickly, return the mixture to a quick boil and then immediately turn it down to a simmer for another minute or two. Try the test again. You might need to do it a third time. You should not need to try a fourth time. See notes for another freezer test.

Remove the jam from the stove top and let cool for about 30 minutes until it's easier to handle. Give it a stir before pouring the mixture into the jar/s. Store the jam in the fridge for up to one month unopened. Once it's been opened, try to consume within a week or so.