What did you do to unlock the Celeste-Ra'yan-Bergo threesome?
To make Bergo "in love" talk to him while you control Serenai ( To do this specific scene you don't need this part of Bergo, just Ra'yan in this state is enough ), while Ra'yan stays in this "romantic" state when you use him and talk to Celeste. Keep them both close to each other to make it easier, when they are both in this state you first click on Celeste's "heart" with Ra'yan, he will be in that position with her:
Bergo has to be close to them, quickly release control of Ra'yan (without moving the character) and take control of Bergo and quickly hover the mouse near the couple, a similar heart icon will appear for everyone, then you you can join them. I tried to do this with Percival, the icon appears but was illustrative, probably in the future maybe you can do this scene with him.
I've done the story way too many times and have never been able to get a Celeste x Bergo scene.....
In sandbox mode, You (Bergo) talk to Serenai, in this you will be in the "romantic" state, with this you can go to Celeste, click on the heart icon that appears on her and this way you will unlock the penetration scene with her.
In the sandbox mode, acquire the diamond by doing a mission by clicking on that table at the beginning.
Click on the "diamond" and at the end of the cut-scene you will receive it and then you can use it in the bar. Then you can go to Celeste and then you can release the oral scene. At the beginning of the sandbox there is a table with icons, click on the diamond using Bergo, it will drop this item, you use it in the bar. Leave him drinking and go get Celeste and take her to Bergo.
Take control of Bergo and click on her heart icon and then you can unlock the blowjob scene with her.
The same happens with Ra'yan, if you talk to Celeste using it you can release the penetration scene, if you use the diamond with him in the bar and talk to Celeste you can release the blowjob scene with her.
It's similar with Ra'yan, if you talk to Celeste he will fall in love and then you can perform the penetration scene, if you use the diamond he receives on the first day at the bar (or you can use Bergo to get and then you give it to Ra'yan), you release the blowjob scene by clicking on it. If I'm not mistaken, the same thing happens with Serenai, if you're just falling in love, the penetration scene is allowed, if you go to the bar then she masturbates you.
Serenai in a passionate state masturbates Ra'yan and Bergo, while make Percival do a oral on her, Bergo and Ra'yan penetrate her/Celeste in a passionate state.
If Bergo or Ra'yan have sex with Celeste while they are drunk this activates her romantic state, if you are controlling her and click on Percival you will probably do the penetration scene with him, instead of the kiss scene he does with her .
Percival can only be with Celeste if he is "drunk" then you have a kiss scene, then he enters a state of romance and when he clicks on Celeste he penetrates her, however the animation is different from that of the other male characters. He with Serenai only perform oral on her in this mode,
If you use Serenai and talk with Celeste she will enter in this romantic state, find Percival and click in his heart icon and you will able to the first scene she have with him in the story mode, anyway, as I said, the only scene that I haven't been able to do so far is the scene where Serenai penetrates him.