One way is to use itertools.groupby to create an iterator that groups sequences. The list needs to be pre-sorted first. The function compares elements of one sequence with the next using a key: lambda i: (i[0], i[1]).
Then you just have to concatenate the third element summed.
from itertools import groupby
sorted_1 = sorted(list_1)
grouped = (list(group) for key, group in groupby(sorted_1, lambda i: (i[0], i[1])))
summed = [group[0][:2] + (sum(k for _, _, k in group),) for group in grouped]