Just received my Tamron 150-600 mm. To my vast disappointment, it would not focus beyond 300 mm unless I manually brought it close to focus.
However, on a whim, I found that under the 5DIII AF menu, "Lens drive when AF impossible" was "OFF". When I turned it "ON", the lens focused just fine! So, save yourself some aggravation and change your settings before you dump the lens in the box and return it!!