Add the redcurrants to a preserving pan and roughly crush with a potato masher. Add the lemon juice and measurement water, cover and cook gently for 20–25 minutes, until soft.
Stir in the strawberries and raspberries, then cook for 5 minutes, until just beginning to soften.
Pour the sugar into the pan and heat gently, stirring from time to time, until dissolved. Bring to the boil, then boil rapidly until setting point is reached (20–25 minutes). Skim with a draining spoon or stir in butter if needed.
Ladle into warm, dry jars, filling to the very top. Cover with screw-top lids, or with waxed discs and cellophane tops secured with elastic bands. Label and leave to cool.
To serve, this jam makes an excellent topping for individual steamed sponge puddings.