Can I run multiple games at once on a triple monitor setup?

If you have separate GPUs running the screens, might be possible. In some versions of Windows, and with some other additions to other OS, you can run a virtual machine that is its own computer, right inside the normal OS. That way you might have two programs, two screens from two GPUs. But with only one CPU, each would run at half speed at best.
But doubtful on same GPU.

Yes it is possible to run it in fullscreen mode per monitor. However on Windows some applications will move over to the "Primary" display when they are launched into fullscreen mode. I don't know a solution for that (look around) but my guess is that borderless window fullscreen mode is one way to work around it. In other words it depends on the program you're trying to run in fullscreen whether it moves to your primary monitor or stays on your secondary.

One way I can think of is to set the game/program to windowed mode, move it to the screen you want it to be full screen on and then switch it to fullscreen mode (with a shortcut like for example Alt+Enter)

If you're on Linux however (which I doubt with Arma 3), Fullscreen and Borderless Window Fullscreen mode are the same thing, and windows always stay on the screen you put them.

No. The physiological output of your monitor would negate the GPU Input velocity, therefore quantum linking the petroleum gap between your core speeds and the paleontologic central processing unit.