Personally I'd always try to focus a landscape manually, or pick a mid point in your focus area (assuming high aperture of f11 or f16) focus using AF and then switch the lens to Manual Focus to lock it for your shots. That's assuming you're doing this from a static tripod.
I can only comment from my 7D experience (similar but less amazing than the 5D3 AF) and any landscape wide angle images I've taken were basically with manual focus. If you want to simplify the 5D's system, select the zone mode for AF if hand holding. Hopefully I'll be getting a 5D3 in May, so I'll look forward to seeing how you get on with it as I'd itching to get one.