Just to be clear, two of them - a boy and a girl - were normal teens going to school before the entire plot happened and shit rocketed fanwards. These two were dragged into an unknown side of the realm that they were unprepared for, and then summarily had to earn their right to keep breathing by adapting and beating back the darkest terrors of the world with a sword, an old book and a very loosely defined limit on 'acceptable levels of property damage'. The other character - also a girl - is a 500+ year old dhampir that has spent the last few centuries fighting otherworldly monsters in order to keep average people safe out of an instilled sense of noblesse oblige, and has turned into a snarky, lackadaisical sociopath stuck in a teenager's body as a result.
The boy is meant to be a character torn between his two natures, seeking to find balance in this new world and the responsibility that he has been given. Part of him wishes to retain his moral boundaries but it is often challenged, and there are times when his refusal to bend or his refusal to commit to his boundaries have cost lives. The first girl is the heart of the story - someone who tries to stick to their moral code no matter the cost and is perhaps the most paragon of them all. The dhampir is meant to be the foil - someone who had seen literally everything that is happening to the other two and been disillusioned by it all and struggling to keep the two others safe above all else.
Basically, I plan to have all three of them end up in a polyamory. Would you believe it to be a deal breaker for a story if it came to that?