Hunch. Some past reviews that changed nothing left no release, so three or more new members is closer to even than the nine reviews show.
The three-or-more market resolves on the same December review release: YES if it names three or more companies joining the S&P 500, and NO if it names fewer or does not come out by 18 December. The rules on separate and mixed releases and the backups are the same as for the any-company market.
We looked for cases where S&P ran its regular quarterly review of the S&P 500 after at least two companies had already joined early in the same quarter. We started in September 2019, the first review S&P announced two weeks ahead. We found nine such reviews from 2019 to 2026, and counted how many companies each review added.