I've made a comp consisting of three layers. It's just a dotted line (with trim paths) and two arrowheads following the line.
I've brought that comp into my main comp and it looks great.
But now I want that dotted line comp to "follow" the background as it moves (I'll parent it to a null object). But when I turn on 3d Layers for that dotted line comp… it goes crazy! Bouncing around all over the place.
Not enough info. If you are new to AE your workflow is probably doing something like trying to parent a 3D layer to a 2D path. To help we need to know your complete workflow in detail. At least give us a screenshot that shows all keyframes.