I used the following criteria in order to build this cube.

All cards have a converted mana cost of 3. 24 rares, 96 uncommons, 240 commons. Packs are 10 commons, 4 uncommons, and 1 rare. 60 white cards, 60 blue cards, 60 black cards, 60 red cards, 60 green cards, 30 gold cards, 30 colorless cards. Initial wave of cards were chosen with activated abilities that cost three mana. Next wave of cards chosen have "3" or "three" within the game text. Next wave of cards chosen were 3/3 creatures. Final wave of cards were chosen to fill in the gaps and try to not include any other numbers.