Two methods with slight difference in who Ian dates first:
Ian dates Lena, is close to Holly and goes on a trip with her, kisses but doesn't fuck her. Lena takes back Ian while constantly choosing LenaxHolly options. This should lead to Holly kissing Lena who is dating Ian. From there the dialogue options for Ian determine if he accepts them both or not at the beach, leading to a threesome and throuple relationship.
The other option is Ian dating Holly, he can also sleep with Lena but not fall in love, and Lena trying to seduce Holly, leading to Holly kissing Lena while dating Ian. Different dialogue will play out, but the idea is the same. From there Ian determines if he accepts them both or not at the beach, leading to a threesome and throuple relationship