@JR: It sounds like your lens(es) and camera body may be out of alignment. Thats not really all that uncommon (all manufactured equipment has to be manufactured within certain tolerances, and when you have broadly compatible interchangeable parts, tolerances usually have to be loosened to a greater degree than would be ideal), and the primary reason most higher grade cameras like the 5D III include lens micro adjustment features. You may have a general adjustment problem, or it may be lens specific. The 5D III supports adjusting for both cases, however by default micro adjustment applies globally.
I would try micro adjusting your lens+camera combinations and see if that improves your results. You will need a calibration chart or device. For a chart, you might try this one: http://regex.info/blog/photo-tech/focus-chart
(this site also includes very detailed instructions on how to print and use the chart.) If you really want to go all out and get things extremely precise, you should probably get a LensAlign device: http://michaeltapesdesign.com/lensalign.html.
Canon cameras allow you to micro adjust per-lens, and I think you can store up to around 20 lens micro adjustment profiles. The camera will automatically select the right profile for a given lens when that lens is attached (I am not sure if that works with third-party lenses or not...Canon lenses are microchipped with a bunch of statistical information.)
Hopefully micro adjust will help, and prevent you from having to return your camera (and incur all that extra shipping cost and who knows what other costs.)