When all three NR"N were blemished, then they must return in separate reincarnations, and the Ruach and Neshama will ensconce within the Nefesh of a convert.
If a person merited his Nefesh, Ruach, and Neshama, but then blemished them, the three do not return together. Rather, each one returns in a separate reincarnation. But what are the laws governing rectification of each one?
…a blemished Ruach cannot join an already rectified Nefesh
When a Nefesh reincarnates into a body for tikun and becomes rectified, then the Ruach is not able to join with it. As we have already mentioned, a blemished Ruach cannot join an already rectified Nefesh. And certainly a blemished Ruach cannot join the Nefesh before it is rectified because the Ruach can enter only after the Nefesh, which is on a lower level, is completely rectified.
How to escape from this dilemma? How can a blemished Ruach reincarnate at all?
…the Ruach must reincarnate alone with the Nefesh of a convert…
Thus, the Ruach must reincarnate alone with the Nefesh of a convert whose Nefesh is in place of his own, and there it will become rectified. In the same way, the Neshama will come into another body together with the Nefesh of a convert.
The concept of "Nefesh of the convert" has already been mentioned, and it will be mentioned further several more times. A special supplementary section is included at the end of Chapter One, Section 11 and Note 10 there.
This is the secret of what is written in Sabba of Mishpatim (98b): "The Neshamot that encounter the Nefashot of converts … and they merit through them." A Ruach alone, or a Neshama alone, can only enter a body if there is a Nefesh there. Instead (of their own Nefesh) they take the Nefesh of a convert, and through it they achieve rectification.
The blemished Ruchot and Neshamot (plurals of Ruach and Neshama) encounter the Nefesh of the convert, which becomes the vehicle or interface for their entry into reincarnation. "They merit through them" because they achieve rectification through this arrangement. (This entire section from the Sabba was translated and explained in Note 10 to Chapter One.)
Having said this, we can answer an important question. The majority of people only merit their Nefesh. And only a small amount in these later generations ever merit to their Ruach and Neshama. Yet, we know that the son of David will not come until all the Ruchot and Neshamot are rectified (Talmud Yebamot 62a).
The question of the Rabbi is based on the famous statement of the Talmud: "The son of David will not come until all the Neshamot have finished going out from the body." According to Rashi "the body" is the name for the storehouse of the souls. The souls must go out from there to enter into physical bodies in this world in order to become rectified. The Rabbi now relates this teaching specifically to the soul-levels of Ruach and Neshama. If the Ruach and Neshama do not come into physical bodies in these later generations, then how can they become rectified? Mashiach ["the son of David"] would never be able to come, G‑d forbid.
Now we understand that the Ruach and Neshama can reincarnate into other bodies on top of the Nefesh of a convert, and they, too, become rectified.
However, when the original Nefesh eventually achieves rectification, it can receive in place of his own the Ruach of a righteous tzadik whose deeds were similar to his. This will be in place of his own Ruach. …in place of his own Ruach, he will receive the Ruach of a tzadik…
The "original Nefesh" is the one that could not become the vehicle for its Ruach and Neshama because the latter were blemished or damaged in the first reincarnation. When this Nefesh becomes rectified in subsequent reincarnations what happens to it? Its Ruach and Neshama have gone to the Nefesh of a convert. Therefore, in place of his own Ruach, he will receive the Ruach of a tzadik.
Similarly, he can achieve the Neshama of a righteous tzadik.
If his Nefesh leaves this world before the original Ruach has finished its tikun, then the Nefesh can, in the meantime, accompany the Ruach of the righteous tzadik to the World-to-Come and through it, receive its fitting reward . Once its own Ruach completes its tikun through its own reincarnation in another body, as explained above, then the Nefesh will say, "I will return to my first husband," and it will reunite with its Ruach. Likewise, once the Neshama becomes rectified, his Nefesh and Ruach will return to become one with it.
[Commentary by Shabtai Teicher.]