I filed my N-400 under the three-year marriage rule, but my U.S.-citizen spouse and I may divorce before the oath ceremony. Can USCIS still approve me under that rule?

No, not under the three-year spousal provision if the divorce becomes final before the oath. The applicant must remain married to the U.S. citizen through naturalization, and separation can also affect the marital-union requirement. You may still qualify under the regular five-year rule if all of those requirements are independently met, but USCIS must be told about the change.