The input() function returns a string which may contain a "list of numbers". You should have understood that the numbers[2] operation returns the third element of an iterable. A string is an iterable, but an iterable of characters, which isn't what you want - you want to average the numbers in the input string.
So there are two things you have to do before you can get to the averaging shown by garyprice:
Hint for step 1: you have to split the input string into non-space substrings.
Step 2 (convert string to integer) should be easy to find with google.
HTH