This is a fantastic solution. Very well done!

Fantastic one-liner!

Writing a good indented code.

mul = lambda : sum(i for i in range(1000) if not i%3 * i%5)
Use a generator ....

def mul():
  return sum([i for i in range(1000) if i%3==0 or i%5==0])

Look at what the function range() does. Here you iterate from 0 up to 1000 included.
Re-read the instruction. Re-read your code, then you will find your mistake as your program logic isn't wrong.