Based on you description, you have limited possibilities of combinations, hence, have a look at this to ensure that you aren't missing any possibilities
rows = [['A', None, None],
[None, 'G', None],
[None, None, 'P'],
[None, None, None]
]
for row in rows:
if row[2] is None:
row[2] = [row[0], row[1]][row[0] is None]
print("row... {}".format(row))
row... ['A', None, 'A']
row... [None, 'G', 'G']
row... [None, None, 'P']
row... [None, None, None]