JOEL - In LDS terms the three heavens refers to the Celestial, Terrestrial, and Telestial Kingdoms of Heaven (1 Cor 15:40-41, John 14:2). These are the destinations of most of the inhabitants of the earth after the final judgement. Most of what we know about these three degrees of glory comes from modern revelation and scripture.

Celestial Kingdom - This glory comes to those "who received the testimony of Jesus, and believed on his name and were baptized after the manner of his burial,- and who overcome by faith, and are sealed by the Holy Spirit of promise, which the Father sheds forth upon all those who are just and true" (D&C 76:51-53). Within the celestial glory are three levels, and to obtain the highest requires a temple marriage or sealing.
Inhabitants of the highest celestial degree inherit "thrones, kingdoms, principalities, and powers," and dwell with God and Jesus Christ forever (D&C 76:54-70; 132:19-20).

Terrestial Kingdom - This is for those who lived honorable lives on the earth but "were blinded by the craftiness of men" and were "not valiant in the testimony of Jesus." Those who did not receive a testimony of Jesus while on earth, but who could have done so except for their neglect, are also heirs to the Terrestrial Kingdom (D&C 76:72-74, 79). They obtain not "the crown over the kingdom of our God" (D&C 76:79) and remain without exaltation in their saved condition (D&C 132:17). They "receive of the presence of the Son, but not of the fulness of the Father," and their kingdom differs from the celestial "as the moon differs from the sun" (D&C 76:77-78).

Telestial - Within the telestial glory there will be varying degrees of glory even as the stars vary in brightness as we see them. It will contain those who on earth willfully reject the gospel of Jesus Christ, and commit serious sins such as murder, adultery, lying, and loving to make a lie (but yet do not commit the unpardonable sin), and who do not repent in mortality. Telestial inhabitants as innumerable as the stars will come forth in the last resurrection and then be "servants of the Most High; but where God and Christ dwell they cannot come" (D&C 76:112). Although the least of the degrees of glory, yet the Telestial Kingdom "surpasses all understanding" (D&C 76:89).
(Excerpts from Encyclopedia of Mormonism)

A sinner is anyone who knowingly and willfully transgresses the divine laws and commandments of God(1 Jn 3:4). It can also refer to those who refuse to do what is right when given the chance(James 4:17). Everyone over the age of eight years, who has ever lived or will live can be called a sinner, except of course Jesus Christ. Noone is perfect and all of us come short of the glory of God because of the sins we commit. However, because of what Jesus did for us we can obtain forgiveness for our sins and be saved into one of the three levels of Heaven. We are saved in heaven by the grace of God. But exactly which level of heaven we end up in depends on how well we overcome sin and on how we live our lives here on earth.