Playing with three or more types of pokemon require several different trainer/support cards to provide the correct energy in order to attack, such as Energy Search or Roseanne's. These cards end up taking anywhere from 4 cards to 10 cards in your deck (depending on number of types). Futhermore, decks with many types will become even rarer thanks to the new format of DP-on, which removes nice multi-typed energies such as DREs and Scrambles. The best alternative is the multi-energy, and you can't attach more than two on the same pokemon either, or they'll become obselete.