Really clever and great idea! Also amazing job making this in three hours! At first I was a bit confused what I had to do. For some reason my mind thought that I head to combine the red and yellow bullet mid-air to make an 'orange bullet', which could then hit the orange box. It was confused that nothing happened after I did that and I pressed R to restart. After a few tries I figured out I could press spacebar again to make the orange gun shoot, which is when it clicked what I had to do. If you decide to continue development on this game this is something to make more clear I think. (Although for three hours I was impressed that there was a tutorial in the first place, so this is not really a criticism :P)
Otherwise this was a really great idea and very well executed! I definitely think you should continue working on this if you want to :D If you do, maybe some sound effects, screen shake and particles would help a lot for the game feel. Also it took me a few tries to figure out how to rotate the guns. At first I tried to make a cirular movement with my mouse to rotate it. Perhaps something to think about
Thanks a lot for making this! It was short, but I enjoyed it all the way :D