I'll try and simplify things as possible. If you have any issues feel free to ping me.
1.Ian must have been in at least a casual relationship with Lena at some point. Also Lena_Ian_love should be at 2. You can set it to 2 by cuddling with Ian after having sex with him in chapter 7.
2.Ian must be on good terms with Holly. You can accomplish this by going on those first 2 dates with Holly(The one in the cafe at the beginning of the game and the one in the park) You can also ask Holly to come to her book signing trip with her even if you don't end up going. This will set Ian_go_Holly at true.
3.You can form the throuple from two different relationships i.e. IanxLena or IanxHolly. The most straightforward way is IanxLena. Start a relationship with her in chapter 9. Alternatively for the Holly Path you have to go on the trip with her kiss her and start the relationship as normal on a Holly playthrough. Ian will break up with Lena to be with Holly but you should have no problems still creating the Throuple.
4.For the throuple to form Lena_go_Holly must be at least 4. Here are way you can raise it as Lena: By telling Holly she is pretty when she wants to quit pole dancing(+1), by kissing Holly instead of letting Ivy do it t her place(+1), by trying to kiss Holly in Lena's room in chapter 8(+1), by trying to kiss Holly during her photo shoot in chapter 10(+1), by telling Holly you've been thinking about her in chapter 11(+1). By picking the heart option at the cafe when speaking to Holly(+1). In chapter 11, when Holly comes to visit Lena, if Lena_go_Holly is at least 4, i will get upgraded to 5 and Holly will kiss Lena. This will set the flag for the throuple to form.
5.Another thing to note is after recent patches the throuple cannot be formed with slutty anymore

. So make sure Holly doesn't sleep with Ivy or any men if you are playing the IanxLena path. On the IanxHolly path Holly will automatically reject other men when she starts to date Ian. Lena however can effectively sloot around without this affecting the chances of the throuple forming.
6.At the beach house when Holly confesses to Ian about kissing Lena, select the "Just that?" option and express your support for Holly. From here on out it should be smooth sailing until the threesome forms.
Sorry this ended up being longer than I wanted it to be lol.