Question 1:

To sum a list of numbers, use sum:

xs = [1, 2, 3, 4, 5]
print(sum(xs))

This outputs:

15

Question 2:

So you want (element 0 + element 1) / 2, (element 1 + element 2) / 2, ... etc.

We make two lists: one of every element except the first, and one of every element except the last. Then the averages we want are the averages of each pair taken from the two lists. We use zip to take pairs from two lists.

I assume you want to see decimals in the result, even though your input values are integers. By default, Python does integer division: it discards the remainder. To divide things through all the way, we need to use floating-point numbers. Fortunately, dividing an int by a float will produce a float, so we just use 2.0 for our divisor instead of 2.

Thus:

averages = [(x + y) / 2.0 for (x, y) in zip(my_list[:-1], my_list[1:])]

Using zip, sum and list comprehension:

>>> lists = (listOne, listTwo, listThree)
>>> [sum(values) for values in zip(*lists)]
[10, 9, 16, 11, 13]