Pit the cherries. Put half of the pits in a muslin bag and add to the pan along with the strawberries and red currants. Place the sugar evenly on a baking sheet and heat on its own in the oven for 4 minutes.
Add the warmed-up sugar to the fruit, stir until dissolved and bring to a rolling boil. Boil until setting point is achieved (approx. 8 to 10 minutes). Remove the bag of pits. Skim off any scum that may have formed. Pour into jars and seal immediately.