map advice needed

[Trevally.]

With only three line you cant tell the speed or distance.
Just use the three lines to draw the course of the target.

So draw this course ensuring the gap between each is the same.

Now drive for your fourth bearing - you must move after the third.
When the next 20mins is up - stop and take a bearing.
Adjust you drawn course line (parallel) to the fourth bearing.
Only now do you have range and speed