Draw an arrow in 3d space, start and end of arrow flexible

Questions
7

@Evgeni_Popov
so I got the arrow now.
And I Set its origin easily with:
myArrow.position = pickInfo.pickedPoint;

and now I have to set where it points to, and for that it seems I would have to use:
myArrow.rotation = new Vector3(0, 0, 0);

so mmm how would I translate between a 3d gradient direction vector and the rotation angles, I guess that’s the question

I wonder if it would be something related to this:
rx = acos(direction_x); etc
ry = acos(direction_y);
rz = acos(direction_z);
myArrow.rotation = new Vector3(rx, ry, rz);
but not sure…