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I am no expert. I am a low level user just like yourself.
From all of my readings of what you are trying to set up you can run three screens in the manner you currently do for the external view but you can only run one monitor for your cockpit.
There are other comments in this forum on the same subject indicating that only one additional screen can be run for instruments. You need to scroll through all of the questions asked before you. There are several you tube videos on the same subject as well. Try Michael Brown from XForce PC who are the recommended (by Laminar Research) PC sellers of computers at https://www.youtube.com/user/col2mab/videos?sort=dd&view=0&shelf_id=0
However, not knowing what you are running ie a PC with an nVidia Card and windows 7 or 8.1 or 10 try the following. It may work.
In Windows go to Windows Control Panel then Display then Adjust Resolution (side bar menu) then Detect. It should then give you a number of screens. Your three screens should appear. Screen 1 should be your three screens already linked as a single unit. Screens 2 and 3 are the screens for the cockpit instruments. Link these two screens together as one which hopefully will work in the manner you wish.
Alternatively in Control Panel select Nvidia Control Panel then select Display (down the side bar) the select Setup Multiple Displays. From then on it will be the same procedure as in Display.
If as you mention, the GPS is spread across two screens which is a problem. It still mnay do the same when linking the 2 screens. A way around this is to use either of 3rd party software called Panel Builder from Simplugins (they have a demo video and you can download a demo product) or Air Manager. These two products allow you create your own instruments panel and the individual instruments can be moved around for easy viewing. Michael Brown has a number of Youtube videos on this product and advises how to set it up
One thing you need to make sure of, irrespective what you are running, that your triple screen set up should be using the same specifications in all aspects and your twin screens system also has the same specifications. The triple and twin screen configurations don't need to be the same specifications ie you can use different brands,screen sizes and refresh rates.
Hope this helps.
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