as you are using three "if statements" without else if it just runs through every if clause.
in your first if-clause you make label.text to "II"
after that you check if label.text is "II" which happens to be after runnig through firt if-clause.
so just make
if (label1.Text == "II")
to
else if (label1.Text == "II")
and
if (label1.Text == "III")
to
else if (label1.Text == "III")