Preheat the oven to gas 4, 180°C, fan 160°C. Butter and line the bases of 3x20cm (8in) round sandwich tins with nonstick baking paper.
It is best to beat together the ingredients for two of the cake layers first, then the ingredients for the third layer when you are ready to cook it, instead of mixing all of the ingredients at the beginning and dividing between three tins, as the raising agent in the mixture is activated when it gets wet so won't be effective if left to stand for too long (i.e while you wait for the first two cake layers to bake).
First add the plain flour to the ground almonds, then beat two thirds of this mixture, and two thirds of the other cake ingredients, together in a large bowl until you have a smooth batter. Divide equally between two cake tins and level the top of each.
Put in 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 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 to serve. This rich cake is quite a treat, so you'll only need a thin slice!