Does Vi have to deal 3 damage or more or has to deal 3 extra damage?

Based on your question, it seems there might be a mix-up between a few different cards featuring Vi.

1. Vi, Destructive (the Unit card) has the ability: "Recycle 1 from your trash: Give me +1 might this turn." It does not deal damage itself, nor does it have an requirement regarding dealing 3 damage.
2. Piltover Enforcer (a Legend card that features Vi) has the triggered ability: "When you conquer, if you assigned 3 or more excess damage, you may exhaust me to ready a unit."

To answer your specific question regarding Piltover Enforcer: the requirement is 3 or more excess damage.

"Excess damage" is the amount of damage assigned to a unit that goes beyond the amount required to kill it (lethal damage). For example, if you attack a unit with 3 Might, and you assign 6 damage to it, 3 of that damage is "excess damage." The ability checks the total amount of this excess damage you assigned during the combat, not how much damage Vi herself deals.