I've read from a comment in a past chapter that each of the three guy's story loosely follows a tale from Japan they're:
1. Subaru: Kaguya-hime
- Very vague resemblance, when you look into it only the ending match
2. Cosmo: Orihime and Hikoboshi
- Love Triangle-ish? Wherein one is preventing the two from meeting? I know, it's kind of a stretch. Although the ending does fit their ending since they can't really meet frequently considering the situation of their relationship like how Orihime and Hikoboshi can only meet once a year
3. Ginga: Night on the Galactic Railroad
- We'll see how it goes, although I have no idea how they'll do it considering both Giovanni and Campanella are guys