TL/DR It is much harder to get the Liurial 3-some for a hetero player who cheated with Liurial / kept Helanya and a Low/Medium corruption Alexia than was intended due to an unfinished Helanya arc and a mis-coded option. One code fix and the completion of the Helanya event will make it far easier to trigger.
The best temporary player solution is to successfully lie to Alexia about having slept with Liurial and to have Alexia in the mid corruption bracket. That allows for the 3-some without additional relationships ect- just make sure to pass those diplomacy and deception checks .
You could also just fix the code, put Alexia into the medium corruption bracket, and romance X - which is what I did because I didn't want to lie to Alexia
Also- I do say just as a personal caveat: the rewrite of this event is a masterwork. It is easily the hottest 3-some in the game and the detail in the pathing is fantastic. I loved having the option to praise Liurial in the event instead of just degrade her .
And for those of you who are more detail oriented:
You need 5 "threesomePersuadeScore" points in the event for Alexia to accept the 3-some. You can get points from a variety of situations.
If you go negative points Alexia will be enraged at the end of the event. I believe 1-4 she accepts the relationship but not the 3-some- as long as you don't piss her off before you get to the end.
I ended up being 2 points short on my run of triggering the event as well. For a suite of reasons. I'm not sure exactly what your situation in the event was but there were a couple things that prevented me from the getting enough points and if you failed I'm guessing you were in a somewhat similar situation.
1) I cheated with Liurial and was honest with Lexi about it =0 points.
2) I saved Hel and didn't free her = 0 points (for now)
3) I was in a relationship with X'zaratl =1 point
4) No relations with Greyhide = 0 points
5) Two successful diplomacy checks = 2 points
6) Low corruption Alexia and selecting the "we could share her option" = -1 point (I just skipped suggesting it, so I didn't lose this point)
7) No NTR = 0 points
8) Low Corruption Alexia with a miscoded "I swore to protect her" choice = 0 points.
Here are the two things that will make it easier to get the 3-some. 1 fixed coding error and 1 updated story event.
1)The "I made her a promise" choice is coded backwards so both of the choices reward high-corruption Alexia when it is obviously not the intent. So low/medium corruption Alexia doesn't allow getting a point in these choices- when it was intended to grant one.
2) If you save Helenya and kept her you get 0 points for now- but the coding for it looks like you will get 1 point for it based on a trigger that isn't in the game yet - likely pending Alexia / Hel settling their differences. That 1 point also looks like it depends on if you have a positive relationship score with Aleixa. If you have a bad relationship you lose a point.
Here is how you get the most points from the situations in the current iteration of the game. (Brackets for things which I assume will be implemented/changed later)
1) Having not cheated on Alexia with Liurial or successfully lying about it with deception checks = 2 points
* Obviously if you lie to Alexia and fail the deception checks you will fail epically at the event.
2) Saving Hel and freeing her = 2 points.
(Saving Hel and keeping her + Alexia and Hel settling their differences + Positive relationship b/t rowan and Alexia = 1 point)
3) Poly with X'zaratl = 1 point
4) Any Greyhide sexual arc = 1 point
5) One successful diplomacy check = 1 point OR Two successful diplomacy checks = 2 points
6) The "we could share her" option and medium corruption Alexia = 1 point OR for high corruption Alexia = 2 points
7) Alexia having Slept with either twin = 1 point OR Being on a hard NTR path = 2 points
8) When Alexia asks if you will break off with Liurial if she asks you to and you answer:
Of course = 1 point with high corruption Alexia
(I swore to protect her = 1 point with high corruption Alexia but is coded backwards.)
(I swore to protect her = 1 point for medium / low corruption Alexia)