New and Everlasting Covenant Required to be Exalted
“From another revelation to the Prophet Joseph, we learn that there are three degrees within the celestial kingdom. To be exalted in the highest degree and continue eternally in family relationships, we must enter into "the new and everlasting covenant of marriage" and be true to that covenant. In other words, temple marriage is a requirement for obtaining the highest degree of celestial glory. (See D&C 131:1–4.) All who are worthy to enter into the new and everlasting covenant of marriage will have that opportunity, whether in this life or the next.”
(“Kindgoms of Glory," www.lds.org, Gospel Topics - link here)
Exaltation is the Highest Level in the Celestial Kingdom
“To live in the highest part of the celestial kingdom is called exaltation* or eternal life. To be able to live in this part of the celestial kingdom, people must have been married in the temple and must have kept the sacred promises they made in the temple. They will receive everything our Father in Heaven has and will become like Him. They will even be able to have spirit children and make new worlds for them to live on, and do all the things our Father in Heaven has done. People who are not married in the temple may live in other parts of the celestial kingdom, but they will not be exalted.”
*Exaltation: Life in the highest level of the celestial kingdom. Those who receive exaltation will become like our Father in Heaven.
“Chapter 36: Eternal Life,” Gospel Fundamentals, 201 - link here)
The Other Two Glories in the Celestial Kingdom
We do not know much about who will inherit two of the three degrees within the celestial kingdom. However, much has been said about the highest level in the celestial kingdom, or exaltation, because that is where the Father wants all of His children to live (see Moses 1:39). The Doctrine and Covenants teaches that temple marriage is the key to obtaining exaltation:
“In the celestial glory there are three heavens or degrees;
“And in order to obtain the highest, a man must enter into this order of the priesthood [meaning the new and everlasting covenant of marriage];
“And if he does not, he cannot obtain it.
“He may enter into the other, but that is the end of his kingdom; he cannot have an increase” (D&C 131:1–4).
“If a man marry a wife by my word, which is my law, and by the new and everlasting covenant, and it is sealed unto them by the Holy Spirit of promise, by him who is anointed, unto whom I have appointed this power and the keys of this priesthood; … and if [they] abide in my covenant …
“Then shall they be gods, because they have no end; therefore shall they be from everlasting to everlasting … because they have all power, and the angels are subject unto them” (D&C 132:19–20).
(Elder B. Renato Maldonado, "Messages from the Doctrine and Covenants: The Three Degrees of Glory," Ensign, April 2005)