Problem:
You can't run Nvidia's Surround-Physx, or AMD's Eyefinity

Solution:

Go to your app.ini located in your \Documents\iRacing.
Open it in Notepad.
Adjust the following like I have done in the attached "App.ini Settings" using the noted basic math:

fullscreen=0
makes iRacing run in windowed mode
reduceFramerateWhenFocusLost=0
You want to set this to zero so that your frame rate is always at it's highest, no matter what you click on.
windowedHeight= the height of your highest-resoultion screen.
In my case, I have a 1920*1080 screen and a 1280*1024 screen.
These numbers are in the form PixelsWidth*PixelsHeight.
Thus, the largest height number is 1080 (because it's bigger than 1024).
windowedMaximized=0
If the window was maximized, you would still see the title bar and the window frame ... that's annoying. Setting this to zero allows us to reposition the window. With this ability, we will make it so that the title bar and window frame are drawn off-screen, effectively making this set-up full-screen.
windowedWidth= (width of your center monitor) + 2*(width of your side monitor)
In my case, it's (1920) + 2*(1280) = 4480.

The next step varies depending on whether you want your 2nd monitor to be your right-hand view, or your left-hand view.

If you want your second monitor to be your right-hand view, then use this calculation:

windowedXPos= NEGATIVE width of your side monitor
In my case, it's NEGATIVE 1280 (with the value entered as -1280 ).

if you want your second monitor to be your left-hand view, then your windowedXPos will be equal to your side monitor width, plus one pixel.

In my case, with a 1280x1020 side monitor, the monitor's width is 1280, so my windowedXPos would be 1280+1, or 1281

windowedYPos= NEGATIVE height of your title bar
In my case, it's NEGATIVE 26. This was found through trial and error. You can see from my attachment "Title Bar Adjustment", I have the title bar of my windows smaller than normal. There is no need for you to reduce the size of your title bars. What you need to do is start with this variable set at (-26) and if you see a sliver of the title bar running across the top of your screen when racing, come back to your app.ini and make this value larger (like -28 or -30) until you no longer see this sliver.

3) But what about your attachment "In-Game Graphics Options" ?
I didn't mention it yet because all of the essential variables here were changed by you when you tweaked the app.ini.
That said, there are a few important factors here:

--> I have the "Number of screens:" set to 1 because I like that view; it draws the image as if all of your screens were one big canvas. If you desire your side monitors to be drawn independently of the center monitor so that they are displayed with an angled view, you can set "Number of screens:" to 3 AND check "Render each screen separately." If you just set the screens to 3 without checking that box, you've accomplished nothing. Play around with the "Angle between center and side screens:" to see what suits you best. The greater the angle, the further sideways you can see.
--> I have my graphical options turned nearly to a minimum. Running in windowed mode is a harder strain on your computer and mine is old, so I've turned off most of the options so I can still achieve smooth visuals. This may not be necessary for you.


Credit goes to Dan Rasch and John Bodin
https://members.iracing.com/jforum/posts/list/65406.page