TypeError: list indices must be integers or slices, not list. Can you help me?

Here's the code

import numpy
from pandas import DataFrame, Series


countries = ['Russian Fed', 'Norway', 'Canada', 'United States',
                 'Netherlands', 'Germany', 'Switzerland', 'Belarus',
                 'Austria', 'France', 'Poland', 'China', 'Korea', 
                 'Sweden', 'Czech Republic', 'Slovenia', 'Japan',
                 'Finland', 'Great Britain', 'Ukraine', 'Slovakia',
                 'Italy', 'Latvia', 'Australia', 'Croatia', 'Kazakhstan']

gold = [13, 11, 10, 9, 8, 8, 6, 5, 4, 4, 4, 3, 3, 2, 2, 2, 1, 1, 1, 1, 1, 0, 0, 0, 0, 0]
silver = [11, 5, 10, 7, 7, 6, 3, 0, 8, 4, 1, 4, 3, 7, 4, 2, 4, 3, 1, 0, 0, 2, 2, 2, 1, 0]
bronze = [9, 10, 5, 12, 9, 5, 2, 1, 5, 7, 1, 2, 2, 6, 2, 4, 3, 1, 2, 1, 0, 6, 2, 1, 0, 1]

olympic_medal_counts = {'country_name': Series(countries),
                            'gold': Series(gold),
                            'silver': Series(silver),
                            'bronze': Series(bronze)}    
df = DataFrame(olympic_medal_counts)



df[['gold']['silver']['bronze']].apply(numpy.mean)

print (df)

This is the error that sends me

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-15-949694c8056a> in <module>()
     22 
     23 
---> 24 df[[gold][silver][bronze]].apply(numpy.mean)
     25 print (df)
     26 

TypeError: list indices must be integers or slices, not list