Think of it this way, JJ+/AK is 3% of all hands. If someone is stealing 30% on btn, that means JJ+/AK represents 10% of all their opens.

So the probability of someone getting JJ+/AK at least once in 5 steals:
1-(0.9)^5
41%

So there's a 41% chance that 4betting you once over those 5 steal attempts in which you're 3betting them, that it is JJ+/AK.