I traded a [white or red] F block for one -- it seems they come from different types of actions, so a lot of people end up missing one of the 3. Now that I've seen the minigame, I occasionally just leave them in odd places for others to pick up.
When you get the block that has 'swf' on it, if you are holding all 3 they will autoassemble into a game box that you can 'play' - it's a tiny glitch module. Very cute.