Hi Craig,
Having looked at your comment following Rishi's initial answer I am of the opinion that your system is based on AMD ie your motherboard is AMD technology based to take the AMD style CPU. Your graphics card therefor also needs to be AMD suitable.
Like Rishi I am also Nvidia based with my system.
My understanding of what DirectX is about, it is part of the technology of the card. Version 11 is the latest technology. I don't fully understand its functionality. What you need to be concerned about is all of the other specifications of the card particularly consideering how many and what type of screen connections you have ie DVI, HDMI and or Display Port.
If your card can support three screen then use it. It should at least have somewhere in the order of 3 or 4Gb of video memory for best results. Preferrably 4 Gb. As Rishi has mentioned you can't run 2 identical cards in X-plane hard connected to operate as one, ie SLI technology. You don't need to run an additional computer for each screen. Have a look at the referenced Youtube videos below.
An alternative method of connecting 3 screens is using the Matrox Triple Head to Go (TH2G) unit which allows you to connect the TH2G unit to one video port on your card and then connect your three screens to the TH2G unit. There are some good details through a Google search.
One of the problems with multiple screen viewing is a small difference in alignment ie one edge of one screen does not align with the image on the adjacent screen. Matrox includes software to assist in the alighnment process. If you were using a Nvidia card there is software available to compensate alignment as well.
If you haven't purchased your three screens as yet you need to consider the following irrespective of what screen size you want to use:-
1. All screens/monitors should be of the same make and specifications ie brand, size and refresh rate. The faster the better ie 1ms rather than 5ms plus.
2. Make sure the screens have the same pixel size (the smaller the better) and they are at least 1920 x 1080 specifications. See the reference below from Michael discussing as one of his videos, the use of 4K screens/monitors.
3. Subject to pricing (dictates screen size affordability) try and obtain screens that have inbuilt Free-sync, If using a Nvidia card make sure G-sync is available. What this means is that the lagging between screens is dramatically reduced ie the three screens are syncronised with each other.
Have a look at the videos by Michael from X-force PC. This company is the recommended company by Laminar Research for the supply of PCs assembled specifically for X-plane. I understand this is a sideline company operated by curent X-plane emplyees. Michael has produced a number of informative videos on several aspects of setting up your hardware dedicated to X-plane. Here is the link https://www.youtube.com/user/col2mab/videos?sort=dd&view=0&shelf_id=0 Michael also discusses using/running X-plane on multiple computers/ hard drives instead of networking an additional computer.
In addition have a look at videos from Wycliffe Barrett at the following link https://www.youtube.com/user/wycliffeb/videos He has some interesting videos. In particular have a look at his video titled "3 Life hacks" (video 142) produced about 3 months ago on the backup of X-plane and running the software on an additional hard drive in your machine instead of networking an additional computer.
Consider upgrading your RAM to 16Gb. Michael has some comments about RAM in one of his recent videos.
Hope this all helps.
Glenn