To solve your problem next time, add lots of print statements.

In this case, extra print statements could have shown you:

Summing over 3 numbers; 1 2 3
Adding 1 Total is 1
Adding 2 Total is 3
Adding 3 Total is 6
Summing over 3 numbers; 2 4 5
Adding 2 Total is 8
Adding 4 Total is 12
Adding 5 Total is 17
[download]

If you had seen that, the bug would be obvious, right? :)