They have. More than thirty years ago, Cadbury's launched Top Deck (or Triple Deck), a bar with a layer of milk, plain and white chocolate. It has had another name at various times too. I think it is still available, but maybe only in Ireland these days.
Frys also had a range of bars with tripple decks. Milk Sandwich was milk chocolate with a layer of plain in the middle (no white, I grant you) and Plain Sandwich was - you guessed - plain chocolate with a layer of milk chocolate in the middle. Then there was Mint Sandwich - with a mint layer.
