Combining (or Mixing) is a method in the game where one can "combine" three of their cards in hopes of getting a better card. The three combined cards have to be the same card type (eg, all heros or specials ). The resulting card from the combiniation is random. Generally, combining better type cards will result in better cards, but this is not always the case. The card made from the combiniation can be better or worse in card type than the cards combined (eg, 3 heros made a plat or 3 plats made a special).
The only way to guarentee a similar card type outcome is by using 32 stars. This is no encouraged for anything less than combining platinums, otherwise it is a waste of stars