Writing a code with 3 digit numbers while they cant contain 0 and then sum them

[perfringo]

In order to randomly pick from all three-digit numbers shouldn't you start with 100 instead of 111?

You should also be aware that random.randint(a, b) returns a random integer N such that a <= N <= b This means that random.randint(111, 1000) can return 1000 (which is not three-digit)

You could take advantage of random.sample():

>>> cisla = random.sample(range(100,1000), 3)

Part of your problem is:

>>> a=[random.randint(111,1000)]    # you create list with random int in it
>>> a
[433]
>>> str(a)                          # you created string from list containing one int; didn't assign name
'[433]'
>>> a                               # a is still list with random three-digit int in it
[433]
>>> if 0 in a:                      # there can't be 0 in a as element because there can be three-digit integers only

In you code cisla is list of lists, something like: [[457], [817], [844]]