We need to sketch in the section shapes. For a simple field you need to draw in a polygon that encloses the whole field.
In this example first sketch in a shape enclosing whole field. Then draw in next line from top right to bottom left on diagonal. Finish with line from top to this line.
For the 3 sections click on lines that define the outline. When all selected right click and then click on the add field menu item.
We have the 3 sections as shown below:
Alternatively enter your surface points manually, through Total Station or import from another package.