The threesome is consensual since all three involved parties consent to it. Lena is absolutely able to choose not to bring up the subject at all, and the whole idea comes from her.The threesome with ian/lena/louise is not consensual then. Lena feels she betrayed Ian, so that 3some is just a consolation prize.
There is no open relationship path in this game, period. The "experimenting" is limited variant of what people think "open relationship" is, and mistaking one for the other is on them. The game never refers to arrangement Lena/Ian make as "open relationship" (on the surface; variable naming is another matter altogether)I feel the open relationship path in this game is neglected and damaged the most by the dev.