[spoiler=route guide] At some point during chapter 3 ( sometime it will happen at chapter 2 for Yuela's route) you will be awoken at night and can either go hit on Yuela or go back to sleep. If you hit on her you will be locked into Yuela's route.
Regardless of your choice, you will later be asked by Serawi to come with her to see something really important (corruption), but you already had plans with the other two girls. If you choose "Serawi is more important" and go with her you will start the Serawi route, but only if you did not already start Yuela's route; if you did start Yuela's route you will go with Serawi and learn about corruption, but will go no further with her, remaining in the Yuela Route.
If you chose to turn down both Yuela and Serawi, you will automatically be pushed into the Emilita route.
If you already hit on yuela during chapter 2... you're "locked" on her route