Is it possible to play a movie from an external hard drive (either Time Capsule or a drive connected to Time Capsule) on Apple TV 3 directly, without going through iTunes on a Mac?
In that case, is it possible to leave a movie on an external hard drive and have it play on AppleTV, while iTunes is open but without it streaming to the Mac and then back to the AppleTV?
Answers (above) to your (OPs) questions are correct. It sounds like you are wanting a solution that unchains your computer from the equation but still works. Think server instead of hard drive. If the server can run iTunes, it can get you pretty close to what you want.
If you have some old computer that you are no longer using, it can be your "server" (if it can run iTunes). If not, you could buy a cheap windows machine (even a used one to get the price low) and dedicate it to this purpose.
OR, consider going with something else apple:TV like) such as the WD device that allows local storage of your choosing.
As the others have already stated, no, it's not possible directly.
However, I should add something: in addition to full x86 computers, you can also use low-power iOS sources (even older ones like the iPad 1 / iPod touch 4) to act as a proxy between a UPnP (and prolly also SMB) NAS and your ATV.
I've thoroughly tested UPnP and it worked just great. All this without any power-hungry, noisy etc. desktop computer.