how to draw a 3D line in catia ?

If you want to draw a 3d line then use, Reference elements-Line and use the context menu, there are lots of option available as per need.

for 3D curve you have to use spline command in wireframe/GSD workbence. first define the points and through these point a 3d curve can be generated.

i will try

If we draw a 3dlline we must select the reference elements line and select the plane and extrude it