Keisuke's and Jun's routes are fairly linear. There are some choices that lead to different flavor text, but nothing significant.
Shoichi's route has a split into 3 different versions.
To get route Route A you need to choose:
Day 21: Tell the truth / Deny it
Day 22: “What I said?” / “I still mean it.”
Day 22: “Ignore your father!” / “Agree with him for now.”
Day 24: Maybe a little / Not at all
Day 24: I can’t bring myself to stay silent / I need to be quiet
Day 26: Be firm / Be apologetic
Day 26: Keep pushing / Let it go
To get Route B you need to choose "Let it go" on Day 26.
To get Route C you need to choose "Deny it" on Day 21, "Agree with him for now." on Day 22 or/and "I need to be quiet" on Day 24. After that "Be firm" and "Keep pushing" on Day 26.