Preheat the oven to 350 degrees. GM3/ 170°/ 325°F
lubricate and line 3 x 6-inch circular baking tins (note that they are smaller than standard tins; if using 8-inch tins, see my instructions on scaling up the recipe)
In separate bowls, melt the three varieties of chocolate. Set aside some time
Using electric beaters, whip the butter and sugar till light and fluffy in a mixing basin. One at a time, add the eggs, beating well after each addition.
Continue to beat in 3 tablespoons of milk. Sift the flour and baking powder into the mixing bowl and well combine.
Into three small mixing dishes, divide the cake batter evenly.
To make the White Chocolate & Vanilla Sponge, take a tablespoon of the cake batter from the first mixing bowl and immediately stir it into the melted white chocolate. Gently stir the white chocolate mixture and vanilla extract back into the remaining cake batter in the mixing bowl. When the chocolate is stirred into the cake mixture, this double-action prevents it from seizing. Spread the batter out with a blunt knife in one of the baking tins.
Pour an extra tablespoon of milk and the Ovaltine powder into the second bowl of cake batter for the Malted Milk Chocolate Sponge. Take a tablespoon of the cake batter from this bowl and immediately whisk it into the melted milk chocolate. After that, fold the chocolate mixture back into the cake batter. Pour the batter into the remaining baking tins and spread it out evenly.
Pour an extra tablespoon of milk and the cocoa powder (sifted) into the remaining bowl of cake batter for the Dark Chocolate Sponge. Mix a tablespoon of the cake batter into the melted dark chocolate, then fold it back into the remainder of the recipe. Fill the last of the baking tins with the batter.
Bake for 20-25 minutes, or until the cake springs back when lightly touched or a skewer inserted into the center comes out clean.
Allow for 2 minutes of cooling time in the tins before removing from the pans, peeling off the baking parchment from the bottom of each sponge cake, and cooling entirely on a wire rack.