Hi pplmeir,
I am a "simmer" just like you. I have no connection with Laminar Research.
There are comments previous to yours where LR research have advised that 3 screens cannot be used as standalone units.
However it can be done provided your graphics card has a minimum of 3, better with 4, monitor or tv screen ports. You don't need to have independent computers. The trick is to link your 3 external view screens as one unit. This can be done through Windows - Control Panel - Display.
The alternative trick is to link the 3 screens as one through NVidia graphics card display. If you want good performance stay away from AMD graphics cards. In saying this, others have made this comment from experience.
In relation to your display units, ie either monitors or TV screens, and whatever size you can afford, make sure they are exactly the same model and size, The only difference will be the serial number of each unit. If you don't them you will be wasting your hard earned cash. If you chose to go for a large screen over and above 27 inch then television screens will be a lot cheaper. Whatever screens you chose also make sure they are at least 1920 x 1080 format. X-Plane operates at its best with 1920 x 1080 resolution.
Whatever you do, don't contemplate acquiring a 4K resolution TV or monitor. You will waste your money. The best place for a 4K unit is in your normal television viewing area. This has been stated by others who, as part of their business, set up flight sim systems.
For further information have a look at the following link. Michael is the Principal of a company called XForcePC. This company is recommended by Laminar Research as the preferred supplier of X-Plane systems within the USA. The link for the informative Youtube videos is https://www.youtube.com/user/col2mab/videos?sort=dd&view=0&shelf_id=0 In these videos, Michael also explains and demonstrates the use of a 4th screen as your cockpit instrument panel. This is the way I will eventually go.
The short answer to your question is - it can be done using one computer. Trawl through Michael's videos.
Hope this helps.
Glenn