Yep, it's got asked already, pretty much a FAQ

Apparently those 3 are connected, a chained events or whatever u wanna called it
To put it simply :
- Corrupt both Airi and Momoko up to....I dunno, prolly >50% each
- During afternoon go to Momoko's room and take a peek (unlock Momoko #4)
- Keep it going with Momoko inside her room (unlock Momoko #18)
- Keep it going with Momoko until Airi entering the room and decide to join the fun (unlock Airi #24)
ps : if there's no Airi's follow up scene (#24), try it again with Airi's corruption higher than Momoko's