The incarnations you meet in the Fortress are not there because they reached it during their lifetime. Only the Practical Incarnation did of those three.

1. The Original Incarnation tells you the crystal split you upon touching it;
Original Incarnation: "I am one of your incarnations. I was once lost, now I am here again" ... "Whatever you touched within the Fortress has brought pieces of yourself to the surface."

2. The "Sounding Stone" that the Practical Incarnation made also tells you this is what the crystal does;
Sounding Stone: "As for the crystal... it doesn't seem like it belongs in the Fortress, and its size and shape reminds me of something I've read in the annals, some sort of dimensional prison that segments the soul."

3. The Original Incarnation ponders why those three specific incarnations are there;
Nameless One: "I have had countless lives. Why are there only three incarnations here?"
Original Incarnation: "I do not know. Perhaps we were the three pieces that were somehow still present in your mind."
Nameless One: "Present? How?"
Original Incarnation: "I do not know for sure, but I would guess that when we die, traces of the former personality may remain in your mind..."

4. The Original Incarnation tells you he's never been to the Fortress before. Even though he did lie to you about some things, it's unlikely this was a lie considering #5.

5. The Original Incarnation couldn't possibly have gotten trapped in the Fortress considering;
a) The Transcendent One set up the crystal trap in the Fortress.
b) The Transcendent One did not exist until Ravel's ritual.
c) Ravel "killed" the Original Incarnation right after she made him immortal, as we learn from Ravel;
Ravel: "Not long after the spell a-drew to a close, I killed you to see if it had worked. You struggled so, but I kept my grip *tight* and watched you die your first of many deaths." Ravel *clacks* her teeth. "Then was I a-learned in its flaws..."
Nameless One: "What flaws?"
Ravel: "Ego enwraps us like a prison. Forgot I did that it ofttimes serves as a shield." Ravel clicks her tongue. "My pretty, pretty thing, there is much wisdom and understanding in the truth that life is a preparation for the ultimate goal: death. Our life is a means by which we learn *how* to die. If we FORGET such things..."
Nameless One: "So that's when you discovered I lost my memories when I died..."
Ravel: "Yess..." Ravel nods. "Unfortunate... without the mortality to hold such memories tight, the shell a body is..."

And from the Original Incarnation himself;
Good Incarnation: "I found that changing my nature was not enough. I needed more time, and I needed more life. So I came to the greatest of the Gray Sisters and asked her for a boon - to try and help me live long enough to rectify all the damage I had done. To make me immortal."
Nameless One: "And Ravel did. But when she first tested your immortality and killed you, you forgot everything. *Everything.*"
Good Incarnation: He looks broken. "And the Planes have been dying ever since. The crime is great, and the blame is mine."

6. Aelwyn tells you she killed what was probably the Paranoid Incarnation;
Aelwyn: "You and I, Aelwyn, have met on two different occasions [...] the last more recently. Perhaps no more than fifty years ago [...] You lashed out at me, Aelwyn, made to slay me. Screamed how I, Aelwyn, could not fool you, would not ensnare and murder you [...] We had met in one of the northern towers of the Festhall, on the seventh floor. Before you could choke the life from me, I, Aelwyn, used my powers to bade you leap from a window to your death. When I, Aelwyn, finally went in search of your broken body, you had already gone..."

7. The Practical Incarnation tells you;
Practical Incarnation: "I was the first to breach this Fortress"

8. Chris Avellone has said "... the three incarnations were chosen just because of the Rule-of-Three in Planescape and because they were the ones that had the most impact on the player's life throughout the game." (source)

Go back to Platter's Miscellaneous Torment Crap.