You are correct, that is how it works and it is excessively deadly.
You can alter the effect if you like of course, I suggest the first failed saves makes you fatigued, next failed save you become exhausted, the hp to zero, -1, then death.
A young Void Dragon is CR 9.
Suffocation is a 5th level spell:
This spell extracts the air from the target’s lungs, causing swift suffocation. The target can attempt to resist this spell’s effects with a Fortitude save—if he succeeds, he is merely staggered for 1 round as he gasps for breath. If the target fails, he immediately begins to suffocate. On the target’s next turn, he falls unconscious and is reduced to 0 hit points. One round later, the target drops to –1 hit points and is dying. One round after that, the target dies. Each round, the target can delay that round’s effects from occurring by making a successful Fortitude save, but the spell effect continues for 3 rounds, and each time a target fails his Fortitude save, he moves one step further along the track to suffocation. This spell only affects living creatures that must breathe. It is impossible to defeat the effects of this spell by simply holding one’s breath—if the victim fails the initial saving throw, the air in his lungs is extracted.
So the worst interpretation - that the breath weapon is a true save-or-die - is not necessarily out of line with intent.
You might house-rule a couple of options:
1. You could allow the flat Constitution checks every round (10 + 1/round) which are normally allowed to characters when they run out of breath-holding ability. This seems to be consistent with the idea that you *could* suffocate for longer than three rounds.
2. You could allow the saving throws every round to avoid advancing on the suffocation track, with the total duration being (the dragon's age category) instead of (3 rounds).
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Warning: rant.
I hate when a developer or contributor make something that he thinks is a clever idea and never bother thinking about how it interacts with other stuff or give clear rules on how it works.
Suffocating Breath (Su) Instead of dealing cold damage, a very young or older void dragon can breathe a coneshaped suffocation effect. An air-dependent creature that fails its Fortitude save suffocates (Pathfinder RPG Core Rulebook 445) for a number of rounds equal to the dragon’s age category. The save DC is Constitution-based.
What is "An air-dependent creature"? Seem straightforward, but it covers someone that is using elemental shape or undead anatomy? An amphibian? Someone with Water breathing running? Burrowing?
Those are all things that allow you to live without breathing air, some only in specific circumstances. A term that has no specific meaning as far as rules go was used as if was self-explanatory, but it isn't self-explanatory in a world where magic works and is used constantly.