The Reference RText Command
Ron Couillard of USCAD demonstrates how you can employ the Express Tools RTEXT command to externally reference raw text files in AutoCAD and Civil 3D.
From the USCADBuilding YouTube channel
and Ron Couilard's blog on USCAD.com
There are some other juicy CAD Commands the Rock in Ron's post.
Where you locate the external Text file(s) in your project gets to be important.
If you prefer to employ Excel references, there are posts and training videos about that start in Not So Simple Tables in Civil 3D to learn more.