OK, I've done a little more digging, and I'm very confused. I wish I could read a decision or a memorandum that specifically addresses this point, but I haven't found one. I'm leaning toward the theory that the three years of marital union means you have to have lived in the same house with your spouse, but it's only required for the three years prior to the examination date, that is, the date they finally get around to approving your application, not the date you first send the paperwork in. But I'm very confused and unsure about that.
Anyway, here are the laws and regulations
INA 319(a) lists the 3 year requirement
QUOTE
(a) Any person whose spouse is a citizen of the United States, or any person who obtained status as a lawful permanent resident by reason of his or her status as a spouse or child of a United States citizen who battered him or her or subjected him or her to extreme cruelty, may be naturalized upon compliance with all the requirements of this title except the provisions of paragraph (1) of section 316(a) if such person immediately preceding the date of filing his application for naturalization has resided continuously, after being lawfully admitted for permanent reside nce, within the United States for at least three years, and during the three years immediately preceding the date of filing his application has been living in marital union with the citizen spouse (except in the case of a person who has been battered or subjected to extreme cruelty by a United States citizen spouse or parent), who has been a United States citizen during all of such period, and has been physically present in the United States for periods totaling at least half of that time and has resided within the State or the district of the Service in the United States in which the applicant filed his application for at least three months.
8 CFR 319.1 clarifies this
QUOTE
(a) Eligibility. To be eligible for naturalization under section 319(a) of the Act, the spouse of a United States citizen must establish that he or she:
(1) Has been lawfully admitted for permanent residence to the United States;
(2) Has resided continuously within the United States, as defined under §316.5 of this chapter, for a period of at least three years after having been lawfully admitted for permanent residence;
(3) Has been living in marital union with the citizen spouse for the three years preceding the date of examination on the application, and the spouse has been a United States citizen for the duration of that three year period;
(b) Marital union--(1) General. An applicant lives in marital union with a citizen spouse if the applicant actually resides with his or her current spouse. The burden is on the applicant to establish, in each individual case, that a particular marital union satisfies the requirements of this part.
Two important points: One, "marital union" means actually residing together. But second, the three year marital union requirement is for the three years preceding the date of EXAMINATION, not the date of filing. I've been able to find several court cases that talk about couples separating (but not yet divorcing) after having filed the N-400, but shortly before being naturalized, and both of these points are clear when the time spent living apart is immediately before being approved for naturalization. But in my case, I'm concerned with time spent living apart at the beginning of the period, 3 years before filing.
INA 334(a) establishes the 90 day "head start" provision that lets you file before you've finished the continuous residence requirement.
QUOTE
(a) An applicant for naturalization shall make and file with the Attorney General a sworn application in writing, signed by the applicant in the applicant's own handwriting, if physically able to write, which application shall be on a form prescribed by the Attorney General and shall include averments of all facts which in the opinion of the Attorney General may be material to the applicant's naturalization, and required to be proved under this title. In the case of an applicant subject to a requirement of continuous residence under section 316(a) or 319(a), the application for naturalization may be filed up to 3 months before the date the applicant would first otherwise meet such continuous residence requirement.
It's curious to note that the statute says 3 months, not 90 days, not that this distinction is so important here.
8 CFR 334.2(b) clarifies this a little, but doesn't add much info that would be relevant to our case.
QUOTE
(b) An application for naturalization may be filed up to 90 days prior to the completion of the required period of residence, which may include the three-month period of residence required to establish jurisdiction under section 316(a) or 319(a) of the Act.