You mentioned portrait and landscape....for these uses, your 5d3 would be a better camera than the 7d2...and comparable to a 1dx. IMO
Good advice, if all your intended use is Portrait & Landscape I don't think the 1Dx brings a lot to the Party, well, it brings a lot, it just may not be useful.
If your intention is to shoot a reasonable amount of action/wildlife/sports, then the 1Dx is the way to go.
If you don't mind carrying a Brick, go 1Dx, if you like to shoot in all weather conditions without using a Condom on your Camera, go 1Dx, if you want the ability to spot focus on any focus point, go 1Dx.
The IQ between the 1Dx & 5DMK III I feel is pretty well the same, it's becomes about, cost & use.
If cost isn't an issue and you don't mind carrying the extra weight, then it's no contest, 1Dx.
You might think about the Sony a7r, I imagine this Camera will be a reasonable Landscape/Portrait Body, you can use your Canon Lenses etc, lighter than either the 1Dx or the 5DMK III, but it's not a Canon so may not get too many votes in this Forum, but Sony make good gear and this Camera may work well, this is conjecture on my part, I don't own one, yet.