Slice the strawberries and rhubarb if using fresh fruit. Set aside.
Mash and cook raspberries till they're soft and juicy. Strain through several layers of cheesecloth. Twist into a ball and SQUEEZE to get all the juice. Scrape the outside to get all the pulp. You don't need clear juice since this is jam. Skip this step if you don't mind the seeds.
Put the strawberries, rhubarb, raspberries or strained juice and sugar in at least a 5 qt pan. Add water if needed to prevent scorching.
Bring to a gentle simmer and cook to 220F on a candy/jelly thermometer or test for jelly by putting a couple of tablespoons on a saucer and letting it cool. Stir frequently.
Ladle into hot canning jars. Cover with hot lids and bands.