I'll be blunt. This is somewhat disappointing for a game with "3" in the title. I didn't experience any glitches, so that's a plus. The controls worked fine, and the game was certainly playable.
The problem was in the game design. There's only so much fun you can pull from eating people, and once the shock value of being a piranha wears off, you're stuck with floating around a lake for 60 seconds, bumping into a boat, and dodging a shark if it happens to show up. The worst part was when I was given the mission "find 2 treasure chests." I found the chests, and then had another 60 seconds to sit around, because I'd devoured every person on the level during my hunt for treasure. Either the spawn rate for boats needs to be upped, or the number of them raised, so there's something to do. An 'end level' button would also be handy, if I didn't just miss it. I'm also concerned about the lack of enemies, in the levels I played. The shark was far too rare to be an actual threat, and the jellyfish were no trouble to dodge. The eel was a nice touch, and caught me off guard the first time.
You did do quite a few things right, however. The upgrade system seemed to be well-thought out, and I was glad to see that we could build up a piranha swarm if we felt like it. The art was also pretty good (though, on a related note, there were some issues with the smaller piranha swimming backwards for a decent length of time, depending on the angle of your directional change. You might want to tinker around with it.)
All in all, a decent game. Not the best game in the genre, but certainly not the worst.