Your deck can only contain cards from exactly three different elements. However, your mark may be any element, including the banned Death, Darkness, and Earth. Having to use cards from exactly three different elements (one from Group A, one from Group B, and one from Group C) means that the least you can have is a trio. If you consider having your mark an element whose cards you do not have in your deck, I suppose that could be called a quartet, but it just depends on one's vocabulary.