In my opinion the particular method for ownship constantly moving is too heavy on the workload of plotting. It is easier to use the idea from US Navy Maneuvering board manual.
When ownship is steady on course and speed, just plot the bearings from the same point, as if ownship is stationary. By using the stationary 3-bearing method, you'll get a relative course or relative motion of the target.
Then you can just use the stationary 3-bearing plus one method to triangulate the target position and get the relative speed of the target.
The only additional thing to do is to convert the target relative motion and speed to true course and speed, by drawing a simple triangle:
Much simpler. I also recommend trying out the historical 2x3 bearing method used by USN from 1941 to get a grasp on relative motion plotting.