Preheat oven to 180ºC conventional or 160 ºC (fan forced). Grease bundt pan with melted butter and a brush, then dust with plain flour, tapping out any excess.
Make the base batter by placing the sugar and softened butter into a large bowl and beat with an electric mixer on medium for approx. 5 minutes until light and fluffy.
Add in eggs, one at a time, mixing well in between each egg. Lastly add in the vanilla and mix for a further 15 seconds.
Split the batter into 3 equal portions into 3 separate bowls (approx. 400g for each colour).
To make the Strawberry batter: in a bowl, combine flour, baking powder and salt and mix well. Add this to one third of the base batter and add in the sour cream, strawberry topping and pink food colouring. With an electric mixer, beat on medium for 2 minutes until a smooth cake batter is formed.
To make the Chocolate batter: in a bowl, combine flour, cocoa powder, baking powder and salt and mix well. Add this to one third of the base batter and add in the sour cream. With an electric mixer, beat on medium for 2 minutes until a smooth cake batter is formed.
To make the White Chocolate batter: in a bowl, combine flour, baking powder and salt and mix well. Add this to one third of the base batter and add in the sour cream and melted white chocolate. With an electric mixer, beat on medium for 2 minutes until a smooth cake batter is formed.
Into the greased and floured bundt pan, start with scooping the chocolate cake batter into the pan first, then scoop in the white chocolate cake batter, then scoop in the strawberry batter on top of that.
Tap the cake pan down onto the bench three times and bake in oven for 55-60 minutes until a skewer comes out clean.
Remove cake from oven and cool in the pan for 15 minutes before turning out onto a cooling rack to cool completely.