I got an order to make two yellow digger cakes for twin boys, and their mom wants each to be about 3kg size. Please any idea on how to make this cakes?.

have you tried searching the gallery? http://www.cakecentral.com/modules.php?name=gallery&meta=search&type=full&search=digger
think about breaking it down into the basic shapes - a rectangle on its side will be the front, a rectangle standing tall for the back/ drivers cabin, so you could cut squares/ rectangles and stack them to the right heights.
id probably go for gumpaste to make the actual bucket on the front. wheels could be just shaped fondant, or if your using buttercream, cake or biscuits could be covered too.
xx

Thanks for the idea, I will surely try it out. Please do you know what size of pan that could give me a 3kg size of cake?.