Understanding Exclusion Logic: Cohorts A, B, and C Explained

Hey! I had a question about using not in logic - getting myself confused with De Morgan's logic. When building a cohort, I want to exclude anyone in who is in any of the cohorts A or B or C. Which is the right logic to use: (User not in Cohort A Cohort B Cohort C) - I get 1,688 users returned. (User not in Cohort A) and (User not in Cohort B) and (User not in Cohort C) - I get 435 users returned. I suspect it's the latter which is correct but wanted to see if anyone else has any ideas...