What is worst case scenario A top of foundation wall is at grade B top of foundation wall is 3 inches below grade C top of foundation wall is 16 inches above grade?

3" below grade.

To make this more understandable, sketch it on paper.

Being at grade is level with the soil, with no sloping. So put a G and a line for "at grade" and draw inches above the top of the foundation to see how it looks:

16 inches down to 11 inches == TOP edge of the Wall. ( Has nice casement Windows, providing natural light. Plenty of accommodation given for bushes and plants without interference to Windows.)

10 inches down to 6 inches

5 inches down to 0 inches or Ground

G ======= AT Grade (Ground), level with the soil== \\Soil grading downward away from structure.\\

1 inch below

2 inches below

3 inches below ............. The top of your wall would be lower than the outside ground. The foundation could have no Windows; they'd be useless even if placed way up at the top. Therefore it would get no natural light. Soil would continually erode at the top edge, meaning, over the years the foundation would appear from the outside even lower than what it is with being below grade. Worse-- no one will be happy with appearance or function.