Q: If I file my N-400 90 days early, can USCIS approve me before my five-year anniversary?
A: No. The 90-day rule lets you file early; it does not shorten the five-year continuous-residence requirement. USCIS cannot naturalize you until you actually reach the required five years as a permanent resident and meet the other requirements.
Source: USCIS Policy Manual, Vol. 12, Part D, Ch. 6: https://www.uscis.gov/policy-manual/volume-12-part-d-chapter-6