The D3 will balance better with those lenses than the D700 without the grip. Getting the grip would be opposite of what you want, since it would then be possibly heavier and larger than the D3.
I find my F5 balances better with the pro zooms much better than my F100. On my D200, I use the grip.
If it was me, I would keep the D3. You will take a loss if you sell it and it is the better camera.
There is no real reason to save some weight with the camera body if you are using heavy lenses in my opinion. I choose to use the heavy lenses, so I don't notice much difference in weight with the slightly ligher cameras.
Shoot nature with respect and don't trample it or startle its inhabitants. :)