df = pd.DataFrame({'A': [1, 2, 3], 'B': [4, 5, 6], 'C': [7, 8, 9], 'D': [1,
    3, 5], 'E': [5, 3, 6], 'F': [7, 4, 3]})
How to create list of 3 or 4 columns of Dataframe in Pandas when we have 20 to 50 colums?