You can use Dual Display Mouse Manager. It's used to lock your mouse to a single screen until you drag the mouse for a certain time or press a defined button. If you just want to exclude the third monitor you can set the button to something rarely used and set the region to span both monitors.
Sounds like you want to check out "DDMM - Dual Display Mouse Manager"
It lets you configure when the mouse can move onto the next screen, only when the ctrl key is pressed and/or after you keep the mouse trying to move over for a time.
I've been using Dual Display Mouse Manager for a while now. It works like a charm. Only thing is that sometimes you have to restart the program after enabling the mouse management before playing a game (only takes a couple seconds). Just look at the little icon on your task bar, the monitor that your mouse shouldn't go on will be white, that means it's working. http://ddmm.sourceforge.net
Weird, I have dual monitors too but haven't encountered this problem with Stalker.
I do have a workaround for you though: http://ddmm.sourceforge.net/
Using this program you can capture the cursor on one screen.
I use a third party program called DDMM: http://ddmm.sourceforge.net/
I've been using it for about a year or two for this exact issue. It halts the mouse from going to the other screen, and you can keybind it so if you hold a button (ex: Ctrl), then you're able to jump to the other screen while holding the button. Love it.
I set it to load as the M1 key on my keyboard to load up quickly whenever I have a game that needs it.
Thanks. I ended up using DDMM and find that to be working well locking the mouse to one display. If only there were an app that could make two independent mice work on dedicated displays. That would be the best scenario. So I can browse on one screen and play an another with independent mice; mostly for playing/selecting music off foobar2000. Without the nonsense of turning off displays or ctrl+alt+del/select taskbar etc.
I personally use http://ddmm.sourceforge.net/. It will lock you mouse out from switching monitors until you want to. You can set it up by either holding your mouse to the monitors edge for a bit or using the ctrl key to switch between, helps out a lot.