IMHO , you can file N-400 90 days prior to 5 th anniversary of GC, but there cannot be an Interview held before completing 5 years of GC. By any chance , if one files 90 days prior and gets his interview date , say 30 days prior to the 5 th year anni., then the officer will have to reschedule the Interview to a date so as it is after the 5 th year anniv.
One simply cannot be interviewed before completing 5 years of GC.