Synergy – 2 computers, 3 monitors. middle monitor for its own pc

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 :)

Adapted from ArchWiki, here’s an example that should provide enough of a reference:

section: screens
    Pi:
        switchCorners = none +top-right +top-left 
        switchCornerSize = 40
    Desktop:
        switchCorners = none +bottom-left +bottom-right
        switchCornerSize = 40
end
section: links
    Pi:
        up(0,100) = Desktop(25,75)
    Desktop:
        down(25,75) = Pi(0,100)
end
section: options
    keystroke(Control+Alt+KP_Multiply) = lockCursorToScreen(toggle)
end

This incorporates the following features:

• The upper side of Pi’s screen is mapped to the center 50% of the bottom of Desktop’s screen and vice versa.
• By pressing CtrlAltNumblock * you can toggle the “lock cursor to current screen” feature.
• A 40-pixel area around the corner of the marked edges is protected and keeps the mouse inside the current screen. Pointless for the “Desktop” screen of course, just kept it as an example.

You might need to adjust the percentages to match your actual set-up.