Neonider 3 is certainly your best entry into the series. Polished Geometry Wars-esque graphics, solid controls, and the addition of missions was a plus. Though, being the third in the series, I would have liked to see more variation from the previous installments. There are also a few problems I ran into.
First off, the power-up collection missions took way too long, especially as they starting reaching a goal of 30. By that time, most of the power-ups are upgraded about halfway, and they take a bit of time to expire. Being that you can't collect any while one is active, this eats up far too much time without there being any kind of progression in the game. Even waiting for only health powerups takes forever. I think that making health power-ups drop every once in a while no matter what is active would have been helpful.
I also ran into somewhat of a bug, or at least an inconvenience; While the coin magnet is activated, if any part of the red collection field goes over top of the pause button in the top left corner, you cannot click it until you move it away. It's like the collection field blocks it. When you start getting larger and larger fields, this can be quite annoying. I know you can hit "p" to pause also, but sometimes you can't really stop to hit it while your avoiding fast moving objects.
Finally, I'm dreading that the first secret medal is to reach 1 million points.
Looking forward to what you have to offer next!