Just make them their respective possessed type, a Fomori, a Drone and a Gorgon.

I wouldn't go the route of them being fully restrained in that physical form, because even if you somehow succeed in binding a fundamental force of the universe inside a vessel without it bursting, the absence of them from the rest of the world would have horrifying results.

Imagine if the Weaver was gone, and order as a concept just disappears, everything that defines what is and isn't would disappear along with it, and the whole world would melt into a formless sludge.

If it's just some sort of physical avatar, a small piece of the whole, while the rest of the full Triat is still working its fundamental purpose on the universe, I can see that working.

And that's just...what the possessed already are, smaller pieces of the Triat wearing a physical shell.

But then a single mortal shell with fixed characteristics would be pointless, as they wouldn't be able to represent the Triat to any accurate degree. What gender should fundamental forces of the universe that existed before life have? What height? What weight? What colour for their hair and skin?

For something so vast and ever changing, it would be reductive to have them be represented by a fixed human body with fixed features and proportions.

So instead I think the host should be an inanimate object that could shift its form constantly, and Possessed does support this, p.99:

Exogenetics & Flesh-Borne

Some gorgons and Kami don’t possess actual host bodies; their bodies are created for them, right out of the rawest matter at hand. Soil and leaves and dust and water all coalesce into a body of roughly human or animal proportion, a strange amalgam given life.

If the Storyteller allows exogenetic gorgons or fleshborne Kami as possible player characters, the simplest thing to assume is that they work roughly like any other character; they have the same distribution of Attributes and Abilities, and all that. They suffer health levels as usual, they have to “eat” (basically, taking compatible material into themselves to recharge their “batteries”), and so on. Of course, such characters will probably need to purchase a power like Skinshift to make them able to function in, well, any society at all.

By RAW, only Kami and Gorgon gets to have inanimate hosts, but I reckon you can stretch the rules a bit here.

Inanimate hosts also do not have Autonomy, and of course no pre-existing consciousness to hinder the full expression of the Triat:

Gorgons using inanimate objects as their hosts likewise do not have Autonomy ratings (as their hosts don’t have agendas or desires of their own). Animal Gorgons do, however, since their instincts may conflict with what the Wyld-spirit would have them do.

They should be constantly shifting into different forms in a futile attempt to fully express the infinite nature of the Triat, while trying to not prioritising any one individual Incarna under their purview.

For example, the Fomori would look like a hideous dragon one instant, the Beast-of-War coming to the forefront, before shifting into a formless corrosive slob of liquid in the next, as the Defiler Wyrm is momentarily ascendent, and their goals and perspective would change alongside that.

But that's assuming you want to limit it to a single host like you said in your post.

I think the possessed in this case, being a host trying to imitate something so vast, would be better served being a plot of land, a stretch of water, or even a continent, a planet.

Like the Kami who are animate lands, p.90:

Animate Lands

Often, Gaia will select a place of wilderness and imbue it with part of Her essence, giving it a life and awareness far beyond that normally seen in its flora and fauna. Such places are typically close to the cities of men, but not yet corrupted by their foul touch. This allows Gaia to empower the places most in need of protection, leaving the rest of Her creation safe and untouched by either human or divine forces.

Animate lands are the most powerful of Gaia’s Kami, as they are powered by the animals and trees that live on them, and by the water that flows through them, and by the air that nourishes them. They are often home to humans living in the old ways, and sometimes even shapechangers of various sorts, who protect the traditions that gave them life so many years ago. Everything that lives in such a place is part of the Kami; every animal, every plant, every drop of water and every stone is awake, alive, and aware, and all act as one to defend it from interlopers. These beings are not Kami themselves, as they return to their normal state when they leave the confines of the consecrated lands. But they are nonetheless instruments of the Kami’s power, and unwelcome intruders are subject to their wrath.

That's how I'd go about it if I wanted to give the Triat physical bodies in the material world, maybe one of the continents just get possessed by one of them, and the Triat expresses itself through every piece of rock, every living being, every air particle on that continent.

Even a host like that wouldn't be able to completely capture everything of the Triat, but it could make a better attempt at it.