You know, in order to check for a winner, you must check for full (that is having the same symbol)
rows,
columns or
diagonals.
I give you an example (checking rows):
def check_row(board, row):
cntX = 0
cnt0 = 0
for i in range(row*3,row*3+3):
if board[i]=="X":
cntX = cntX+1
elif board[i]=="O":
cnt0 = cnt0+1
else:
return ""
if cntX==3:
return "X"
else:
return "O"