Preheat the oven to gas 4, 180C, fan 160°C Butter and line the bases of 3x20cm (8in) round sandwich tins with nonstick baking paper.
Beat two thirds of the cake ingredients together in a large bowl using a stand mixer or an electric beater until you have a smooth batter. Divide equally between the tins and level the top of each. Put the oven and bake for 30 minutes, or until a skewer comes out clean and the sponge is golden. Make up the remaining mixture and cook it as soon as the others are out. Once the sponges have cooled for a few minutes, tip onto a cooling rack and leave until completely cold.
Meanwhile, make the buttercream by beating the ingredients together. Once the sponges are cold, put one on the base of your presentation plate or stand and spread with a little buttercream, followed by the strawberry jam. Slice the strawberries and arrange them on top. Add another layer of sponge, then spread on more buttercream and the next flavour of jam followed by the blueberries. Add the final layer of sponge, then top with the remaining buttercream, smoothing it out or making it wavy. Top with more berries, but don't overdo it, or it will be hard to cut! Dust with icing sugar if you like.