Any Way To Run A New 3" Pipe INSIDE An Existing 6" Pipe & Have The 3" Pipe In A Fixed Position Inside
There are products called "pipe casing spacers" which can be used for such a purpose.  I'm not meaning to endorse this particular brand but this is an example of such a device:

Pipe Casing Spacer

A drawback, however, is that the original line will be significantly blocked by the combination of the new pipe plus the spacers, so if you need to maintain flow in it, what you get will be quite a bit less than what you currently have.