I have a question. My son is trying to do this in a game and I'm saying you can't.

He wants to use 2 mystiques and three con artists to perplex domino's damage from a 2 to a 7. Can you do that? or does the rule of three kick in?

Perplex modifies one of the five values, and Damage is one of those, so you can only gain a +3 to Damage from Perplex.

However, do not confuse Damage value with Damage dealt, which is not capped. Damage dealt is increased by (to name a few):

Objects
Critical hit