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.