You will be blown away by the sharpness and tonality attainable with medium format in general, and with the Contax 645 system in particular.

As far as the D3 is concerned...well, you're asking the wrong question on the wrong forum! [Better whisper it to me or else you'll get smacked.]

In the Contax system, I'd go for the 140 first as it's a superb portrait lens, one of the best. It stays on my contax 90% of the time. After that I'd look at the 80 or the 55, depending on what you usually shoot. The 55 is very hard to find. If you need more background isolation the 210 is available and is under-rated, in my view.

If you need wider than this, I'd skip the 45 and go for the 35. It's one of the best very-wides around.

So the short answer is, without a doubt, yes, YES, YESSS! you will benefit by adding the contax system, or any medium format system.