Draw the outer and inner contour in one go. Then draw the output line all at once (first straight line, then arc, then straight line). Then create with the new ladder command. What you need to do here is create the ladder, edit it, and then enter 3 in the balanced number of steps at the beginning and end in the fields tab. The ladder you want will be formed. In fact, if you look at the attached picture, you will see that the staircase in question is among the ready-made stairs.