3D motion paths are not impossible in Motion. Here's an example: https://www.youtube.com/watch?v=99UvA6yJ7Gw It is however, not a single step process; but in many ways, easier and more flexible.
The basic deal is this: you draw a 2D "map" of the "trip". On a map, this would be creating Bezier line from point to point — destinations. The fastest, best way is to do this from the top looking straight down (along the Z-axis) and "flat". Adding a camera and changing angles will give the greatest 3D effect.
Once you have the map path drawn, you can add Behaviors > Basic Motion > Motion Path behavior to the object or the group. I also recommend using Snap Alignment to Motion behavior. (Set the initial alignment of the object to line up with the path, within the nearest 90 degrees.)
You may have noticed that once you add a Motion Path bahavior, the Position and Rotation parameters of your object become "locked" — not a problem. To have an aircraft-like object take off, gain altitude, pitch/yaw/roll (climb/bank/turn), deviate from the Motion Path is to add "Custom" parameter behaviors. From the Behaviors menu (or the Library) > Parameters > Custom. Add one for Rotation.All. From this behavior, you can keyframe takeoffs/landings, banks, and other types of flying motions...
To control the airspeed of your plane, in the Motion Path behavior, change the Speed parameter to Custom Speed and keyframe the slider (which is % along the path, very much like Last Point Offset for a shape's outline.)
Other things you can do are: add Random Motion to the Position (turbulence), add Oscillate to the "roll" axis (usually Z, but depends on your plane image orientation) or more Custom behaviors as needed.
All in all, it's easy, you just need to know about the Custom behavior that is essentially an "Offset" behavior, particularly in this case.
