Sorry for not posting here sooner. I'm just catching up on my month long near absence, and it took me a while to work down this far in the forum. I hope you don't mind a few thoughts after so much time.
What's to fix? I like the premise. Let's explore and play a game of what if instead.
Now, brace yourself for a long ramble that will raise many questions while answering none. If I get carried away, my thoughts might turn out to be longer than the story you are trying to write
If it's too daunting, just skim down to the last section. It's usually the last paragraph or two where I sometimes manage to make sense.
Proceed at your own risk.
You have been warned!
You have created an interesting, yet somewhat familiar setting to work with that could actually go in several directions. You have the "Light" (Lux), the "Dark" (Obscurom), and the "Neutral" (Umbrae), In a setting of a lost and forgotten history, which gives you a lot of flexibility. I assume from your description that Earth's current cultures evolved from these three cultures, but the history of that evolution has been lost. That leaves you free to borrow inspiration from most any ancient culture on earth. It only stands to reason that some of their traditions would have been handed down to modern day, despite the loss of the historical origins. Who is to even say that these cultures originated on Earth to begin with? They may have well migrated from Mars when the ecology collapsed, rendering the planet the cold desert it is today. Or maybe they WERE here all along, and mankind is far older than any of us ever imagined. Who is to say? Oh, that's right; YOU are to say! Such is the beauty of a lost history. It leaves you free to invent your own history.
My first impression here is that the Lux and Obscurmom would be naturally opposed to one another, with the Umbrae exerting something of a moderating influence. A buffer so to speak between two opposing cultures. I would imagine that no one faction would be thrilled with their Princess aligning with any one other faction, and the realtionship between these three women would not be something their peers would encourage or be comfortable with.
What you have here is a three legged stool, which is not exactly a stable platform, especially if all three legs are not equal in their goals, design, or purpose. You have an atmosphere that is by its very nature prone to instability, intrigues, and misunderstanding. In a word, that would be a triumvirate. A modern example of a functioning triumvirate would be America, in that it has three equal branches of government. The Executive (Presidency), Legislative (Congress), and Judicial (Supreme Court). One writes the law. One interprets the law. And one upholds the law. They are often opposed to one another, yet must work together in order to establish the laws we live by. Take away any one of these, and one of the remaining two would subvert the other. It is a weak and cumbersome form of government, but it is constructed that way on purpose. No one branch can function without the others, and the presence of the others keep any any one branch from gaining absolute power. In your OP you insinuated that one faction had designs that might not be to the best advantage to the remaining two. That creates a broad range of possibilities. Some rather messy and complex possibilities.
I find it something of a coincidence that you have chosen a Latin influence for your work, since it is from that era one of the most famous, successful, and ultimately tragic triumvirates took place. here is a little tidbit you can google around with for inspiration if the mood strikes you.
The First Triumvirate was a political alliance between three prominent Roman politicians (triumvirs) which included Gaius Julius Caesar, Gnaeus Pompeius Magnus (Pompey the Great) and Marcus Licinius Crassus.
The Second Triumvirate is the name historians have given to the official political alliance of Gaius Octavius (Octavian, Caesar Augustus), Marcus Antonius (Mark Antony), and Marcus Aemilius Lepidus, formed on 26 November 43 BC with the enactment of the Lex Titia, the adoption of which is viewed as marking the end of the Roman Republic.
Interesting how the combination of your Latin setting and three character theme have a couple of real world counterparts, eh? Even more interesting is the fact that both of those stories are alike, yet have dramatically different endings.
Now that I have bored you with history, let me take off in a more entertaining direction. You have described a setting that could be a fantasy, sci-fi, alternative history, mystery, tragic, uplifting, romantic story. Seriously, you could go about anywhere you want to go. Just as an example, this strikes me as a good premise for a story-reveal. I like a good story-reveal because it begins at a certain point in time, then slowly reveals the past or back story a bit at a time, which has a direct impact on the end story. Just as an example of somewhere you could go, let me take your outline and spin it into a premise. I'll stick with what you described with the exception that I'll use the Dark-Light-Neutral analogy for my own convenience to substitute for the faction names you developed. Bear in mind that this is not a suggestion of what you SHOULD do. It is simply one of many examples of what you COULD do. Do with it as you will.
Three chapters introduce the three main characters in turn, with a later chapter bringing them together in the same time and place. Neutral feels that all factions can live in harmony, and would be better off if they could resolve all the differences and work together for mutual gain. Dark is distrustful of Light and regards Neutral as a hopeless dreamer, but senses Neutral could possibly be a bridge between their people. In time she actually comes to like both, and becomes hopeful that perhaps Neutrals dreams might not be so hopeless after all. Light readily befriends Dark and Neutral for she also wishes a reconciliation where all people could thrive under one banner. The problem is that she has little respect or admiration for either of them. It is her belief that the world would be a better place if all people were unified, so long as they were unified under her people. For the benefit of all; All should be led by those most capable, and her people were the only ones capable of providing the leadership needed. This establishes the conflict. A conflict which two are totally unaware of... at first.
While seeking common ground, and exploring future possibilities (behind their parents backs), the three come to the conclusion that in order to better understand how to overcome their differences, they would first have to understand how those differences came to be in the first place. This voyage of discovery eventually leads them to uncover a forgotten history. A time so long ago as to not be believed, the three factions had lived together as one people. They were a mundane people with almost no magic at all, but were a greater more advanced people due to their great technologies. Although they were one people, they were divided over how those technologies should best be used. One side felt that they should live naturally and use technology as little as possible. The two other sides were convinced that certain technologies were dangerous, and each tried to subvert the other to their beliefs. Eventually these differences divided the parties apart, and that division led to the great war where dangerous technology was turned to dangerous inventions of war. It was one of those inventions, we know not whose, that split the earth asunder and changed the fabric of reality itself. This was the day that technology died, and magic was born.
This awful history was not all grim, for it gave them insight on how to restore the earth to its natural state, to bring the people together once again. The key was there, but it had a cost. To become one people, it would come at the cost of that one thing they had all come to depend on. To use the key... magic... must... die.
Of course, there ARE other solutions as Light discovered, and chose not to share. If her people possessed the weapons of old, they would rule the world. They need not use the weapons. Simply having them would force the others to comply. The world could retain it's magic, and still be unified. Thanks to a dusty old tome that she alone possessed, she knew that not all the weapons had been used. She knew that they were of a design that could withstand the centuries. And most compelling of all, she had a pretty good idea of where thos weapons might be......
What's a Princess bent on world domination... errr... unification to do?
Yeah, I know. Far fetched.
I was up late. Blame the coffee!
Good luck with your story.