This is a bit of design that happened organically, but it has served me nicely. It can be summarized like this:
In DCL, if there is a numeric threshold — from one game state to another, usually —, that threshold generally switches over at 3.
Some examples:
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You win the game by scoring thrice.
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Your success threshold in a round — when you can either push ahead in the race or at least level up — is 3 XP. I blame this as the actual rule-of-three origin: once you set it like this, 3 becomes a yardstick for the Reward stat for party members, and it's easy to then use it as a yardstick for the other stats!
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You can have up to three 'lives' at any one time — your opponent wins by defeating your two Starting Divers, plus any one Power-Up you put down to protect them.
Once I became conscious of what I was doing, I let this yardstick guide more design. For example, these were conscious:
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You have to pass the turn when you exceed 2 risk spent — that is, as soon as you play cards of 3 risk or more.
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1 level is immediately available; 2 requires effort; 3 lets you play almost every card in your deck without having to accumulate more or rely on danger one-offs.
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Your 'at-rate' minion or ally has 3 HP. This means that an attack for 3 damage, undefended, can one-shot most minions.
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This is also a yardstick for how Diver HP is picked. Taki can withstand two to three 3-damage attacks; Titan, three to four.
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This influences all decisions about ATK as well. A character with 3 ATK can deal one unit of significant damage.
This means for Heart that attacks usually will be +0/+1 ATK, because as the emblem of personal growth you need to power up and assemble your best version of the character (Taki starts with 1 ATK and you power them up for another point, then equip the Liminal Blade for a third, for example — at which point each attack card in your hand can one-shot something of regular 3 HP size).
For The Many, Titan starts at 3 ATK and your objective is to play the game with your minions until you get around level 3, at which point the Diver can actually attack.