You ask whether a person who has once been married in the Endowment House or Temple, and is left a widow or a widower, commit[s] adultery by marrying again when the ceremony is performed by a justice of the peace or civil officer. There is a manifest impropriety, which every Latter-day Saint who has had his or her endowments should perceive, in such a person going to a civil officer to have a ceremony of marriage performed. The fact that such a person does so is an evidence that he or she is falling away, because if in good standing a recommendation could be obtained for the Temple, where the ceremony could be performed according to the order which God has instituted. But a person marrying under such circumstances does not commit adultery.
You ask some other questions concerning how many living wives a man must have to fulfil the law. When a man, according to the revelation, marries a wife under the holy order which God has revealed and then marries another in the same way, he enters into the new and everlasting covenant, and so far as he has gone he has obeyed the law. I know of no requirement which makes it necessary for a man to have three living wives at a time.
[Source: Wilford Woodruff to Samuel A. Woolley, May 22, 1888, in Anderson, Devery; The Development of LDS Temple Worship, 1846-2000: A Documentary History, http://amzn.to/TempleWorship]