one issue about having all 3 monitors in portrait mode is that for 90% of you'r flying you will be wasting 33% of your screen real estate (because that will show in cockpit detail like side panels, controls, and dashboard etc) , and those are all wasted pixels your cpu/gpu have to push, hence seriously affecting your frame rates (or your pocketbook if you have to buy more high end hardware)
my personal choice would be to have on large central widesceen monitor directly in front of you, with a smaller monitor on either side in portrait mode. that way you double your monitor total screen area and significantly iimprove your peripheral vision, yet only have to push pixels that are the equivalent of 2 decent size widescreens (at 1920 x 1200).
my intention is to put a 19' in portrait mode on either side of the 27' i am using now, and i still have a spare 19' from yrs ago, so only need to add one more 19' to make this work (which is cheap nowadays). ideally i'd have a 20' on either side because then these dont need to be tilted to portrait mode to match up in bezel and resolution (being 1600 wide by 1200 high), but that means buying 2 more monitors which wont be usefull for anything else and is a bit of a waste of $ for now
using 3 monitors in that configuration looks something like this