Unless your 3D polylines have a serious elevation change in them, linetypes should appear almost normal.
For example, a 1000' long line (dashed linetype) has a visible gap of 10.0000' at 1"=40'. Both ends of the line at the same elevation, and MSLTSCALE = 1.
Raising one end of the line 100' reduces that visible gap to 9.9504', not noticeable to the naked eye.
If you are using the 3D polylines for grading, you might start to see that gap appear smaller. For example, changing the slope of the line to 1:1 reduces the visible gap from 10.0000' to 7.0710'. Still looks like a dashed line though, just with a smaller LTSCALE (0.7 vs 1.0).
As @Simon_Blain mentioned, using a feature line rather than a 3D polyline will keep that gap a consistent 10.000' regardless of slope. Just create a feature line style called Dashed and assign the appropriate linetype to it's display.
I'm still going to stick with my initial response of making sure that all three of your LTSCALE sysvars are set to 1 (LTSCAE, MSLTSCALE, PSLTSCALE).