Why is it always every 3. attack that does extra dmg?

3 is a really common pattern. (2 is also one we use a lot but usually it's with attack resets to get the Attack->Super Attack feeling down right) The reason is that combat times in League (and mobas in general) are really sensitive.
What we're trying to get usually on a 3 hit passive is some amount of commitment from the player. We don't want the effect to trigger off a basic lane trade or harass. However, we also want to make sure the effect is achievable and relying on 4 basic attacks in a row is often borderline impossible in the early game when champions don't have a lot of attack speed.
Later in the game that AS might be available but combat windows have shortened dramatically by that point but the effect still needs to be able to trigger.

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Filip Peterek@fpeterek
Stine Poulsen@Stinebineee