As FLS said, you will spend more time looking up than to the side in an air combat game. Stands to reason then that you ideally want more screen real estate in the vertical than the horizontal. I have experimented with many monitors and I have to say I am really happy with the Acer XB270HU. At 4ms it is not the fastest refresh rate but other than that I really get on well with it and the 144 FPS is nice
My LG 4k monitor looks sweet (LG 31MU97) but things get real small at that resolution and it is caped at 60 fps. I have not tried it with AH3 so might give it a god one day soon
I also have a setup for my car racing and that is 3 x Dell 30" screens (U3014) in landscape mode. I did try AH2 with the screens in portrait and that did give me more of the vertical view I was looking for but the bevels between the screens got in the way sometimes and I found an issue with the clouds / landscape appearing to go backwards on the side screens which I found really distracting.
Just to state the obvious as well, the more pixels you display the more you are going to need a decent video card so make sure that part of the equation is in place as well. I have 2 x GTX980s for my flying rig however I have found no advantage to SLI at all and run on just 1 card now.