Hi marzio288,
Good news and bad news.
Bad news first. If you are running a copy of X-plane on each computer at the same time then you should be running a legal copy of the software on each PC. In your case, as you have indicated, you need three legal licences. This has been mentioned in several places within this forum.
The good news. I am making an assumption you wish to run your three screens as a single external view and have other hardware items for your flight controls. Totally disconnect two of your computers and connect all of your screens to the PC with the i7 7700 processor and the nVidia 1080 graphics card. The card can handle the scenario. Once done then through the nVidia software link your three screens as one. You may have to consult the graphics card manual to do it as I cannot find the link at present.
The alternative is to go through Windows - Control Panel - Display to achieve the same objective. This will show up on one of the side bar links in "Display"
The better way is to use the nVidia connections.
In the meantime have a look at some of the youtube videos provided/produced by Michael Brown at XforcePC, the recommended PC supplier by Laminar Research in the USA. There will be some information you will be able to take on board. The link for all of his videos is at https://www.youtube.com/user/col2mab/videos?sort=dd&view=0&shelf_id=0
In the meantime have a look at the you tube video from Rodger Dodger Aviuation (Matt Thomas). Although Matt is a FSX fanatic whereby some of his videos are relevant to X-Plane. Have a look at the following link showing what I presume you are trying to accomplish. The link is at https://www.youtube.com/watch?v=l7RuOE4w-Bk All of Matt's videos can be found at https://www.youtube.com/channel/UCLURP8iXu35m6ChlVHFsGYQ/videos
What do you do with the other two computers? Matt and Michael give you an option of using Air Manager on one of your machines. Alternatively pull one of the obsolete graphics cards out and insert into your main PC and run Air Manager. Don't link them as SLI. SLI does not work with X-Plane. Browse through their you tube videos for more information about this product.
As a disclaimer, I have no connection with any "party" mentioned in my answer to you. I am just a normal flight simmer like you trying to set up the "best" system possible.
Hope this helps solve your problem
Glenn