If you want to draw a 3D 3 point arc, first align the UCS with the plane passing through the 3 points. If you want to draw a helix, then you can draw a 3D polyline. However, it is unfortunate the you cannot extrude along a 3d polyline, such as to draw a spring.

I am away from my house for another month but for the sake of asking, can a 3d polyline be incorporated into a solid and then modified using the various solid modelling commands?

1) Draw the arc in the x-y plane and use the command rotate3d to position it correctly.
2) Draw a 3D line from the start point to the end point of your arc. Use object ucs and pick the line. Change the view using plan, currrent ucs. Draw your 3 point arc.

The first suggestion - to align the UCS with the three points and then draw the arc - is the simplest and quickest method. It gives you an arc that has a consistent radius (not elliptical) and is easy to manipulate (it can be trimmed etc).

The second message regarding a helix however, describes the situation that often drives one to make a 3d curve. A helix can be modelled using AutoCAD. The trick is in creating a group of nodes to use with a 3d polyline. Once a 3d poly line is used to "connect the dots", the solid>extrude>path command can be used to make a solid helix, like a spring. Making this a "true" curve may not be possible, I do not think AutoCAD will extrude along a spline path. It will extrude along a 3d polyline however. (the distinction to make here is that this method creates a "helix like" object, one that consists of a group of straight line segments, not a true curved object.)

Creating a group of nodes in 3d space can be done using a spreadsheet. Simply make a spreadsheet that calculates the x,y,z coordinates for each node. Import the coordinates using a script file. Once imported, create a 3d polyline that passes through each node. Draw a circle perpendicular to the node at one end. Extrude along a path and select the 3d polyline for the path. A helical solid will be created.

MikeVV gave a very good answer, I just have one thing to add to it.

Before you extrude along the 3D polyline path you can 'fit' the poly line (curve, cubical, or quadratic fit).  Doing this will make the polyline look more like a spline (experiment with the type of fit and number of points used in the polyline to get the results you want - eg: you could draw a 'circle' with the polyline command using say 20 segments, but you may get as good or better results using 6 segments and then using a curve fit).  One word of caution - 'fitting' the polyline essentially adds more points to it, which in turn adds to the time needed for AutoCAD to do calculations with it.

Thanx, I'm trying to draw a compound spring for a shock absorber. I'm much obliged. I did experiment with rotating sections of a torus to obtain a smoother 3d spring section, it wasn't completely successful but it's an approach that other people may find useful for other applications. I'm glad I'm kind of toying with a personal project, or I'd be way past the deadline. HA!! HA!!