Joel - The distraction of persecution, the demands of Church business, and the lack of financial means prevented Joseph from completing and authorizing a manuscript of the translation of the Bible ready for the press.
After his death in 1844 Joseph's widow, Emma, kept the manuscripts and eventually turned them over to the RLDS Church in 1866 who have them to this day. The RLDS Church subsequently made more changes and then printed the first editions in 1867. Later in 1944 another corrected edition was published by the RLDS Church. Since Joseph was not able to see it through to the final publishing of the scripture, many consider it somewhat incomplete in that he did not have the chance to proof read what was eventually printed. In regards to the 1 John 5:7 scripture:

"For there are three that bear record in heaven, the Father, the Word, and the Holy Ghost: and these three are one."

The purpose of Joseph Smith's Bible project was to look for and correct mistakes that were causing confusion in correctly understanding the Gospel of Jesus Christ. He was only working with an English King James Bible and had no access to any of the original Greek manuscripts. Since there was nothing doctrinally wrong with that particular scripture he did not even consider it for correction. If he did not consider it then he would not have received inspiration to remove or change it. Joseph Smith was not infallible, but he did the best he could considering the circumstances he was working under.
The scripture in question is simply mentioning the Godhead in heaven; Heavenly Father, Jesus Christ (The Word - John chapter 1), and the Holy Ghost, and that they are one. We of course understand this "oneness" to mean one in purpose and not one person. In John 17:18-22 Jesus is praying to the Father asking Him to help His disciples be "one" as He and His father are one. He could not of course mean here that He wants his disciples to physically become one human body, but that they work together in harmony as a unit the same way the Father and Jesus do.

These scriptures; Jer. 1:5, Job 38:4-7, Eccl. 12:7, John 9:2, Nu. 16:22, 27:16, and John 3:13, are most often quoted when talking about the pre-existance.
Other scriptures talk about how Heavenly Father is the Father of our spirits, implying that we existed as spirits before we came to earth: Matt. 6:9, Acts 17:29, Heb. 12:9, Psa 82:6, Eph. 1:3-4.
John refers to an incident that occurred in Heaven before the world was created.
"And there was war in heaven: Michael and his angels fought against the dragon; and the dragon fought and his angels.
And prevailed not; neither was their place found any more in heaven.
And the great dragon was cast out, that old serpent, called the Devil, and Satan, which deceiveth the whole world; he was cast out into the earth, and his angels were cast out with him." (Revelation 12:7-9)

Michael is Adam and His angels are all of us who were there fighting with him against Satan and his angels, who were eventually cast down to earth without obtaining bodies.
Jude referred to Satan's losing that war in Heaven as not keeping his first estate. (First Estate = premortal life. Second estate = mortal life.)

"And the angels which kept not their first estate, but left their own habitation, he hath reserved in everlasting chains under darkness unto the judgment of the great day." (Jude 6)

A good site that explains the LDS view of pre-existance is at this site.