5DM3 ordered 3-12 from Amazon arrives tomorrow but I have some concerns.

I think that, unless there's a specific feature that the D800 has over the 5D3, you'll see very little difference in your overall output. The D800 does seem to be technically superior do the 5D3, but by such a slim margin that your output won't suffer either way.

Look at it this way. The camera quality contributes 20% of the image quality, at best . I'd say it's generally more like 5-10%, but that's another discussion. If the D800 is 20% better at "taking pictures" than the 5D3 (also extremely generous), you're going to improve the overall product...4%? For how much money? Given that experience is at least 60% of your image quality, just get 3% better and you'll have the same result.