I see.

In a spoken context, it is easiest to say "One in three [noun] are [something]" and it sounds natural. For example: One in three apples are infected. One in three people are babies.

The correct grammar would be "is". You will hear people say "are", but it's because it's awkward. Think of it this way: "One boy is a good student" "One boy in three is a good student" "One in three is a good student" You'll run into this with "each" or "every" as well, since they often have plurals between them and the verb: "Each of the sailors has a white hat" "Every one of the girls is clever"

I would say "one in three is". It doesn't matter whether they are people or rocks. This kind of expression sounds strange to a native speaker because the last word before the verb is plural (three), but we have to remember "one" is singular.

Not to sure what you mean by the question but, if your asking how to use "one in three" then are would be better. one in three are at the movies tonight one in three are working hope this helps and is what you are wanting to know :)