df2 = pd.DataFrame({'A': [1, 1, 1, 2, 2, 2], 'B': [1, 0, 1, 1, 0, 0]})
df = pd.DataFrame({'A': [0, 1, 2, 3, 4, 5], 'B1': [1, np.nan, np.nan, 8, 9,
    1], 'B2': [1, np.nan, np.nan, 7, 6, 1], 'B3': [1, np.nan, np.nan, 8, 7, 1]}
    )
How to fill in a lot of data utlizing pandas fillna fast?