Here's another solution using statistics.mean() to get the mean of each chunk of every three numbers in the list.
>>> from statistics import mean
>>> first = [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18]
>>> [mean(x) for x in zip(*[iter(first)] * 3)]
[2, 5, 8, 11, 14, 17]
