The three people can end the war if they have access to sufficiently more advanced science, technology, and magic. Clarke's Law says that sufficiently more advanced science seems like magic. And really, really, really more advanced science, technology, and magic can seem like divine power, and make the possessor seem like a god.
If the three people have sufficiently advanced technology they can change the situation. They can remove the causes of the war. They can change people's personalities so they become pacifists and do that on a mass scale, changing all the warriors first and then everyone else.
They can do almost anything you could imagine a god might do to stop the war.
And if the three people don't have access to science, technology, and/or magic that is sufficiently more advanced than that of their world, if they don't have godlike abilities, then their attempt to bring peace is doomed to failure. It is totally impossible for them to stop the war.
At least that is what it seems like to me from your description. And that is the way you want it to seem to your readers at first. Your protagonists must stop the war, but cannot.
But remember, the creator god of your fictional universe is all powerful. 752
So if the creator god of your fictional universe is smart enough he, she, or it can figure out a way to make stopping the war seem totally impossible at first, driving your heroes to despair, while they actually do have a way to stop the war if they think outside the box enough.
Once the creator god of your fictional universe figures out a way for the war to seem unstoppable but actually be stoppable if your heroes can see the method in time, the creator god of your fictional universe can do whatever it takes to make the unstoppable war actually stoppable, and can make sure the heroes figure it out. Because the creator god has infinite power within the fictional universe. if the creator god is smart enough to figure it out, he can plan the whole course of events before he, she, or it creates the fictional universe.
For example, the creator gods of the fictional universe of the movie Only the Valiant (and the novel it was based on) put a long, narrow, and impassable mountain range in New Mexico, with only one tiny narrow pass to cross through, and put it between the hostile Apaches and the US cavalry and the settlers they protected. So a few soldiers could try to hold the narrow pass and prevent the Apaches from crossing.
So maybe the creator god of your fictional universe built such a mountain range with only one narrow pass, and maybe one side in the war has pushed back the other side until the mountain range is between them.
And maybe the creator god made one the peace makers a survivalist who found a cave overlooking the one narrow pass, and set up a pulley system to stock it with food. So the three peace makers steal some wagons full of gunpowder from one of the armies and take them into the pass, and use the pulley system to load tons of gunpowder into the cave, light a very long fuse, climb down, and run out of the pass. The explosion creates an avalanche that buries the pass, separating the two sides forever.
Why don't the armies march around the sides of the mountain range, like the Apaches should have been able to do eventually in Only the Valiant? Because your creator god has made the mountains stretch from sea to sea. Why don't they fly over the mountains? The creator god has prevented them from inventing airplanes or magic flying carpets yet.
Why don't they sail around the coast to attack the other side?
Because 1) the creator god made the seas so rough that all ships sink, or 2) the creator god filled the seas with whales who have recently been hunted and so sink every ship that tries to sail, or 3) the creator god filled the seas with molten lava that would burn up any ship, or 4) the creator go built the planet so the continent and the mountain range go all the way around the equator - like <PERSON> only more so.
https://en.wikipedia.org/wiki/Equatorial_ridge_on_Iapetus[1]
Or maybe the creator god created both the five sources of power and a spell of unmaking that would destroy them. Maybe the three peacemakers go on a long and dangerous quest and discover a source that tells where to find a copy of the spell of unmaking and they go on a long and dangerous quest to find that copy. Maybe the copy of the spell of unmaking, once they find it, instructs them to go on a long and dangerous quest to obtain what they need and take to the proper place (like Mount Doom, for example).
The creator god of your world is all powerful within that fictional world. 164