I can't really tell you how ultra wide + 2x 16:9 works together, although I think AC can do the scene splits just fine and adjust for that!
About the gpu performance:
Google how to activate DSR.
It's in the GLOBAL! 3D settings of the nvidia control panel.
(NOT geforce experience & NOT game specific 3D settings!)
You'll have to do some math and then choose the factors that are closest to the pixel count of your theoretical triples.
In this case:
X*2560x1080 = (2560+2*1920)*1080
Divided by 1080:
X*2560 = 2560+2*1920
Then divided by 2560:
X = 1+(2*1920)/2560
Whoch is 1+1.5 = 2.5
Closest DSR factors:
2.25x and 3.00x
Select only these two in the nvidia control panel and then you'll see two higher resolutions in AC!
Before doing this:
Go into documents\Assetto Corsa\cfg
And make a backup of the "acos.ini"!
That's where the positions of your apps are stored! They are positioned via X and Y coordinates in pixels and when the resolution changes, the positions change too!
Then go on track with the higher resolutions and you'll pretty closely simulate the graphics card load for your planned triple screen setup.
CPU wise, triples make a small difference due to the scene splitting but it's not that bad and your 5600x is a great CPU for simracing!
Your 1660ti will probably have a lot of trouble running it at enjoyable fps.
But just simulate it via DSR! That way you can try to adjust settings to get an okayish mix of visuals vs fps and see if you could get happy