The Third Realm is the 'uppermost' of the Realms. It is the realm of absolute concept and spirit. Everything that ever was or will be exists in the Third Realm, and it is the absolute form of Infinity itself. Absolute Order and Absolute Chaos define the Third Realm. Absolute Ordaos and Chaorder. This is the home realm of outsiders called Youkai, who are borne of infinity. This is also the home of the Aeternalae.
This realm is nearly impossible to describe coherently as rules of time and space are shattered in the face of clashing wills. Indeed, the Third Realm has not physical existence, and it is infinite in boundaries. To know the Third Realm is to know absolute immortality, and if a true God exists, then it is the Third Realm incarnate. A lot of fighting happens here as well, as different factions of Youkai from all manners of species have stark disagreements on absolute truth. However, no death happens here as all souls wind up returning to the Third Realm post-death. So at best, the youkai clashing wind up waging a cold war with each other and engage in the equivalent of mud wrestling with no results. This realm, being a place of absolutes and eternity, can take on the form of traditional Heaven and Hell, depending on the soul interpreting it. Those who are willing to try can experience absolute unity with all souls and final enlightenment can do it here. Without the Third Realm, the First Realm would be without thought or meaning, and the Second Realm would lack any form of definition or energy needed to inspire.
In the Third Realm properly however, it is generally thought that each being essentially lives in an isolated "universe" created as they wish, and interacting with others' if they so desire. When this happens, a "universe" is almost inextricably bound into one of the many subrealms, such as Shinkai and Makai.
In a spiritual sense, if the First Realm is thought of as reality as controlled by external forces, and the Second Realm is thought of as the realm of dreams and energy, the Third Realm is the realm where dreams control reality. As a result, the Third Realm is a very personal experience, uniquely defined by the desires and self-imposed limitations of each individual. For example, if you sincerely want to be an Aeternal, in the Third Realm, you would be. However, if you suddenly decided you wanted to be a Demon Lord, you would find yourself as one. One's personal desires shape their experiences in the Third Realm.
Likewise, self-limitation is an important factor in the Third Realm as well. If you sincerely believe that once you are an Aeternal, that you cannot escape, you will find that you cannot. Likewise, if you believe that you can, then you will find that you can. If you think it is possible to have the knowledge of the universe at your fingertips, then you do. If you think you must work for it, then you must. Indeed, with the immense possibilities that the Third Realm opens to those originally bound by the rules of their particular First Realm, it is quite common for new spirits to exercise no self-restraint, and thus fall into a deep brooding ennui after having everything at their fingertips. This ennui is almost certainly the cause of reincarnation, and a desire to gain the restraint through means other than those imposed by the self. This sort of need that many individuals have for external control (and thus not feel lost in this morphic, personal realm) is why Subrealms like Makai and Reikai exist in the first place.
Philosophy of the Aeternalae strongly recommends people, Aeternal Pledged or not, to practice self discipline in preparation for their arrival in the infinity. Many religions and philosophies can aid people in this pursuit! The better focused someone is, the better chance this realm will fit their needs exactly.