Do note that the D200 is a 1.5x crop camera and the 1DMkIII is a 1.25x crop camera. Canon does not make an 18-200 superzoom, and the closest you will get is a 24-105L. The 18-200 on the Nikon gives a field of view of 27-300, and the 24-105L gives a field of view of 30-131 on the 1DMkIII, so they are very close on the wide end, but not so much on the long end. Also note that the Nikon is a variable aperature lens, with up to f/5.6 while the Canon is a constant f/4.0 aperature.
Portrait and Scenery generally don't fall in the same category. The 24-105L will suffice for both, but the bokeh isn't the best. If you want to lose some focal length, and IS (which is very hard to give up), then you can go with the Canon 24-70L. However, the 24-70L and a 1DMkIII is a HUGE and Heavy package, and wouldn't necessarily bet the best walkaround out fit.
I think Corey gives you excellent advice.
Canon 17-40L for landscapes
Canon 24-105L f/4 IS for walkaround or 24-70L f/2.8 for portrait
Canon 70-200 f/4 IS or 70-200 f/2.8 IS (depending on budget)
That 3 lens kit will be very flexible