Story with multiple 'third planet' races

Question

I'm looking for the name and author of a novel (or possibly novella). I first read this in the seventies, and might be older than that. It might have been written for teenagers.

The story takes place on a planet inhabited by a primitive humanoid race, roughly stone age if I remember rightly. Visiting this planet is a small team from a more advanced race, also humanoid and with spacefaring technology (obviously!). They are either there to help or observe, no nefarious purposes. I think they are avoiding contact with the primitives, at least initially. It's explicitly mentioned that the planet they are visiting is the third one of a medium sized star, just like their own homeworld. Also orbiting round the planet is an even more advanced race, using high tech to remain hidden from both the others. They are observing the whole situation, and again its mentioned that this is a 'third planet', like their own homeworld.

I'm a little bit hazy on the rest of the plot, but I think the super-advanced race end up revealing themselves, even though they are not supposed to.

The story is told from the viewpoints of both the second and third races (maybe also the first, I forget)

EDIT: For the record it turns out my recollection was faulty, and the second race are indeed coming to the planet for nefarious purposes. And the planet is at the medieval stage, not stone age.

Answer 1

There are some intersections with Enchantress from the Stars by Sylvia Engdahl, but I don't recall "third planet" being stressed anywhere, and the lineup of races isn't quite as you describe. I'm going to describe it just in case your memory misaligned :)

I first read this in the seventies, and might be older than that. It might have been written for teenagers.

It was published in the 70s as a "young-adult" novel. The writing style might be a little thick for pre-teens; I re-read it recently and realized I'd been spoiled by the spoon-feeding of modern YA :).

The story takes place on a planet inhabited by a primitive humanoid race, roughly stone age if I remember rightly.

The planet's home race is in a medieval state, with a king and knights mentioned. However, the setting is purely rural; the only interactions with a native are with a woodcutter.

Visiting this planet is a small team from a more advanced race, also humanoid and with spacefaring technology (obviously!). They are either there to help or observe, no nefarious purposes. I think they are avoiding contact with the primitives, at least initially.

The "Federation" (in later books, the "Service") is a multi-race agency of spacefaring observers, committed to watching races grow towards maturity and eventual inclusion in the Federation. There are very strict prohibitions against revelation to immature races, on the lines of Star Trek's "Prime Directive"; agents are expected to die before revealing their nature.

Also orbiting round the planet is an even more advanced race, using high tech to remain hidden from both the others. They are observing the whole situation,

This is where the analogy breaks down. In Enchantress, the third race is of middling advancement - spacefaring, but immature. They have arrived to rape the natural resources and consider the natives primitives without rights.

but I think the super-advanced race end up revealing themselves, even though they are not supposed to

In the end, the Federation agents reveal themselves in a limited way to a native, in order to manipulate him into scaring the attackers away. Finding a limited way - and making it out alive despite it not going as planned - are the culmination of the book.

The story is told from the viewpoints of both the second and third races (maybe also the first, I forget)

Yes, all three races have viewpoints; the woodcutter for the natives, and IIRC two viewpoints ("resolute commander" and "conscience-stricken soldier") for the invaders, and Elena the Federation agent. The majority of the book is from Elena's viewpoint.