When drain pipes under the building are leaking do they need to be
Replaced? Or is there a mid way solution - this is a 6 unit condo built in 1962 and these sewer/main line pipes (3"?) have never been replaced.. some have
Had pieces replaced

There are places that can line them... which is sometimes a cheaper solution. No personal experience with it... I have seen it mentioned a few times here.

As you noted, some drains have been patched and pieced. Why not repipe and eliminate service calls to patch. You can obviously get to the drains to see them. Hard to imaging a 6 unit condo with only a 3" drain. Even single family dwellings in Atlanta are 4". Cast Iron, ABS? PVC is easy to work with, lots of adapters and easy to join to older Cast Iron or ABS drains.