Such as the famous game Plants VS Zombies, or Machinarium. They have Mac and PC version, and even online flash demo. It must be a flash engine or something, does anyone know this, what's that?
Well, for one thing, you definitely can write Flash apps in C/C++.
See this video: http://www.youtube.com/watch?v=0hX-Uh3oTcE
And I suppose with some C macros you can make it cross-compile between 3 platforms simultaneously with some effort.
Other possibilities would including using something like Haxe or the NekoVM sort of thing.