Bezel compensation is disabled by the driver when it detects three non-identical monitors, and there is a very lengthy process to try and make your own INF file for the monitors to trick the driver into allowing bezel compensation.
I didn't even think of searching the registry for any keys like that! It took me two hours to trick the driver into thinking my monitors are all the same when I could have just done that!
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Genius! Did the trick for me even with 3 identical Asus monitors I couldn't work out why this option wasn't available. Thanks!