For those still strugging with the 3 Riddles, I've done some exploring and found the answer for them so you can also enjoy the 3 extra girls:
1st Riddle - Orehurst Town
You will find a flame on the right side of the map where your house is located. There, you'll start a quest to unlock Raven, where you need to find a key to the realm where her body is located. The 3 parts are located in Orehurst, one at the trashcans near the truck; the second near the lake, under a bench; and the last one on the top right of the Clocktower map. After that, you'll just need to enter a neighbor's house and you'll be teleported to a hall, leading up to a maze. Get the book on the bottom left side of the maze (Start from the Top Right portal) and then go towards Raven (Start from the Left portal) to warp and ressurrect her.
2nd Riddle - Town N
You will find a flame on the left street which will ask you to uncover its name through 3 different sets of words. The name of the Strange Ghost is SAUGIRNYN, and in order to find it, you'll need to check the 3 objects they mention: A Mundane (Your TV, a SAUsung); a Grand (the grand clocktower GIRan); and one Divine (The name of a statue in Xerek: NYN). By combining all of them, you'll unlock the path and a new girl: Marine
3rd Riddle - Kinaza Forest
You will find a flame on the left path of the 2nd part of the forest maze, leading down towards a clearing. There, it'll give you a riddle where you have to find it in another part of the map. It's located near the lake, a lonely flower under the shade of a pillar on the left. After you interact with it, you'll just need to return to the forest and enter the clearing, where you'll find Raphtalia.
NOTE: It is better to unlock Raven first, since it'll give you access to a Warp skill, which will allow you to teleport quickly to other locations, and therefore skip a lot of the travelling you'll have to make. In order to unlock a new warp, you'll have to find a pedestal with a blue ball on it and interact with them. There's 4 warp spots in total, one in each important map: Xerec, Kinasa Forest, Academy of Light, and Orehurst Town.