Is there any way to run a brand new 3" water line pipe through the inside of an existing 6" water line pipe. I want to make sure that the 3" pipe is not rattling around inside the 6" pipe. Any way to keep it in a fixed position as well?
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:
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.
If the only reason to do this is to prevent "rattling" of the smaller pipe inside, meaning you are NOT going to use the 6" pipe for anything else, then you can just fill the annular space with expanding foam. It would not be important that it is suspended evenly in the larger pipe, it would only matter than it can't move in there.