Multi-monitors and the corners of the screen

You could use Windows built in tools. On the resolution settings screen, drag the "middle" monitor down slightly in the layout section at the top, so that hitting the top right corner of the middle monitor will cause it to hit an invisible boundary.

Then you'll just need to ensure you hit the top of the screen before going to the right of the screen.

I've done it for mine, and it's worked for me :)

Despite it's limited sounding name, Dual Display Mouse Manager may be exactly what you're looking for. I tried using it for a while but just never quite got used to it. I like to sling my mouse around quickly, so I'd drag a window across monitors and let go of the mouse button only to have it snap back because the software wanted to "pause" my pointer. Maybe it needs to take pointer velocity into account.