1, 12, 27 for Bianca/Hilda/Iris?
1. Who makes the first move and how?
With Hilda/Bianca, they lost contact for a little while in between BW/BW2 when Hilda was traveling and Bianca was busy studying under Professor Juniper. Hilda was the first to reach out after she came back to Unova, but Bianca was the one who officially asked Hilda out a few months later.
When Iris officially joins the relationship, it's more of a mutual decision between the three of them! They were all good friends even before Hilda and Bianca got together, and Iris spends a lot of time with them after the breakup with Cilan. Over time they all realise their feelings are not entirely just platonic, so they all agree to try out dating and see where it goes.
12. What first changes when it starts getting serious?
Things definitely get more complicated where the kids are concerned, especially when Iris moves in with Hilda and Bianca! Athena and Basil get along pretty well, but living together as de facto step-siblings is a pretty big adjustment for two rowdy kids. It's more difficult for Lavender since she and Athena do not get along at all, although she lives with her dad in Kalos so only tends to visit for holidays and such.
27. Why do their friends get annoyed with them?
Poor long-suffering Cheren is just the eternal fourth wheel now whenever they all hang out (maybe even the sixth wheel if Hilbert and N are there). He's probably just glad his friends are all happy, though.