I figured out a way to create the 3D arcs manually.
It’s a multi-step process that involves creating a table of X & Y for all the points of the path you want to turned into a 3D arc, then calculating the Z value (in metres) using a formula: X^2*-1 + D2.
I was planning to create a model for the process.
I’m happy to share the steps with you (or the model if I get around to making it).
I was hoping to find a more efficient way to do this, or even better, an ArcGIS tool to do it for me! I suspect you’re after the something.