We can use ONE and YOU to talk about people in general, including the speaker and the hearer. We only use them in generalisations, when we mean "anyone, at any time".

- You can't learn a language in one month = One can't learn a language in one month
- You need a Visa to enter the USA = One needs a visa to enter the USA
- You shouldn't be too hard on yourself = One shouldn't be too hard on oneself
- For you, your children are always the best = For one, one's children are always the best

ONE is more formal, so it is found mainly in written texts. When speaking, we prefer YOU.

THEY is not so general as ONE or YOU. It refers to a particular group of people, but not a precise group.

- They are very nice in this city  (not everybody in the world, but everybody who lives here)
- Your son is a teenager, and you know that they are very complicated (= teenagers in general, but not everybody in general)
- I'm sure they will sell a new model of this phone next year
- They are building a new bridge over the river
- They are planting flowers in all the city parks
- You work to earn money and they take it away with taxes

We also use THEY to talk about one or more people when we don't know who they are (= somebody)

- They broke my window yesterday
- I went to buy a Keenix camera, but they had bought the last one
- Who made this? If I catch them I'll take them to the police

Compare:

- If you go to Hawaii they give you flowers at the airport

In this example YOU is not "you", but "everybody", and THEY refers to an unknown vague group of people, not to everybody

- What's wrong, are you ill? If you need a doctor, call the emergency number

Here I'm talking about you

- Why am I calling? Well, I suppose that if you need a doctor, you must call the emergency number, so that's why I'm calling.

Here I'm talking about a general situation (and I use it to explain what I am doing). In this case we can also say: If one needs..., one must call...

So YOU can mean "you" or "everybody". The context decides which meaning it has.