Three-reports-to-remove, along with moderator review for the removes, seems to be the golden standard for machine-aided moderation since the days of AOL.

The downside of it is that by the time you have 3 downvotes/reports, at least 3 people have already been exposed to the content. It might not seem like a lot, but that's 3 people who had to read something unpleasant, hurtful, or just a plain old waste of time on your resource.

Minimum 3 readers, usually many more, because people rarely report things, per spam/troll adds up fast.