This is a very ambitious game for something made in three days. Perhaps a slightly smaller scope and a bit more polish would have gone a long way to making this better. As it is now, it is a very enjoyable experience, though there are some aspects that could be improved.
The controls feel very clunky, I'm not sure having to have the player hold down a button to run is a good idea, as there was no reason not to run so just making that the default movement speed would have helped the flow of the game and reduced development effort in having to create two sets of animation for the player character that could have been spent making the jump animation smoother, the jumping felt incredibly floaty. It was hard to tell where the trigger points were for interaction with the npcs which also didn't help.
The game looks nice, the low poly designs for the worlds was a nice throwback to early 3D platformers, and it was nice to have a lot of variety to explore in the three worlds you made. I enjoyed smashing up the rocks, trees, and flowers, but it took me a while to figure out what I was supposed to do with them once I had collected them all. I had fun flying around shooting the UFOs, it was a shame that there was no response to my actions doing so, and the final boss fight was over in less than five seconds, so that was disappointing.
In conclusion, a really excellent attempt for a weekend's worth of work, you are clearly a very talented team. I would be very interested in seeing what future games you manage to come up with.