I've been playing Life With Mary, and it does seem like you can end up in a throuple situation with the main LI Mary, and her best friend Simone.
https://f95zone.to/threads/life-with-mary-v1-0-2-final-likesblondes.7875/
I played DMD a while ago, and I pursued the throuple path with Dee's best friend Elena, because Elena is one of my favourite characters from that game. Although, now I wish I also included Georgina the older female LI just so I could provide Dee with a more stable mother figure in the household. Yes, this is just a game, I know. But I care about these things, lol.
https://f95zone.to/threads/dating-my-daughter-ch-1-4-v1-00-mrdots-games.597/
Triangle, as the title may imply, is also a pretty wholesome throuple situation in my opinion.
https://f95zone.to/threads/triangle-v1-0-neverlucky.18909/
Although I haven't tried this path, I think you can pursue a throuple path with Melody and also her best friend Sophia, so allow me to include Melody here just in case.
https://f95zone.to/threads/melody-v1-00-extras-mrdots-games.10668/