The practice of leaving air space between drywall leaves is known as decoupling, and it’s meant to make it more difficult for sound to pass through walls. Decoupling works best when the air cavities between the leaves are large for better sound absorption. The small air cavities typically found in triple leaf walls act like springs for sound and create resonance. Resonance must be as low as possible for effective soundproofing, and to achieve low resonance, you need a deep air space with a large mass on either side.
At any given size and mass, triple leaf walls will always have a higher resonance point than double leaf walls. As a result, you’ll always get a lower level of sound isolation with triple leaf walls, particularly at lower frequencies.