The first one you gotta start from the beginning and befriend Isaac, if I remember right you gotta tell him the truth.
The second one you gotta talk to Molly and learn she goes to the gym at night on Wednesdays then go work out with her.
The Third one you gotta talk to Brittany at the gym at night (I don't remember if it's like right away or after a few times).
The second Brittany scene takes place in her bathroom. I don't quite remember how to trigger it though .