I'm going with a 3 link front...Radius arm (link) top of axle, and third link just below axle centerline. My question is whether or not it's necessary to center the 3rd link relative to the axle. My mock-ups say no, but I thought I'd throw the question out there anyway.

BTW, I'd only be off center 3-4".

I have a standard 4 link with all 4 links pushed out where the leaf springs would go. I have driven the truck with one of the top links off and it drove at freeway speeds just like normal. And that is with the the single link about 20 inches off center. As long as the heim or whaever joint you are using will allow enough articulation that far out you should be fine, especially as close to center as you are. Later

No the third link does not have to be centered. Most people mount it over the pumpkin.