Writing cubic sums a better way
Instead of three nested for loops, you can use itertools to enumerate combinations. Also, I would personally prefer to keep a count in a dictionary, as opposed to storing the same numbers over and over. 

import itertools


cube_counts = {}

for i, j, k in itertools.combinations(range(1, 500), 3):
    cube = i**3 + j**3 + k**3
    cube_counts[cube] = cube_counts.get(cube, 0) + 1

print(min([cube for cube, count in cube_counts.items() if count &gt;= 12]))