Find every set in a hand of Set cards

Each card in the game Set has four attributes (color, shape, number, shading), and each attribute takes one of three values, so a card is four numbers from 0 to 2. Three cards form a set when, for every attribute, the three values are either all the same or all different. Given a list of cards, return every set.

Start with the direct triple loop, then use the fact that any two cards determine the third card that completes them.