Hello Friend-
If you have a feature line, explode it to a 3dpoly. Then create a block of the 3D Solid with a base point that can attach to 3D poly. Then type Measure in the command line. There you can specify the distance between the blocks that will be spaced along the 3D poly at that increment.
This helpful short video shows this approach in action to create a 3D guard rail. At about 5:50 in the video is where it will relate to your task I assume.
https://www.youtube.com/watch?v=p1GPz4Qrx10
I hope this helps.
Michael M. Carlson
Senior Civil Designer
CADD Manager
AutoCAD Civil 3D Professional
AutoCAD Professional