Q:

how to generate a list of 3-digit numbers?

How can I generate a list of 3-digit numbers for which the sum of their digits equal 17?
For example assuming number XYZ, we would have X+Y+Z=17
This is how far I got:
numbers = list(range(1, 1000))

A:

It appears that you want the sum of the digits to be 17 and not the sum of the numbers as the "but I want the first number + the second number + the third number = 17 ?" implies. 
So, take a look at this:
result = [x for x in range(100, 1000) if sum(int(y) for y in str(x))==17]
print(result)  # [179, 188, 197, 269, ..., 962, 971, 980]

