Please ignore the distinctions of "hello" and "hi" being formal or informal. This is not the case in modern English. They can all be used interchangeably. The context is what makes it formal, not the word itself. You can tell your CEO "Hey, John! So great to see you." This is perfectly acceptable in a formal setting and is common. Hello is not used very much at all in greetings anymore to be honest. Hi is used often when saying "Hi, how are you?" "Hi! Nice to meet you." These are all fine. Pick the one that sounds most comfortable to you and the meaning will be dictated by context not by the word itself.

I would say "Hello" is the most formal, followed by "hi" and "Hey" is the least formal. You use "Hey" when you want to get someone attention. It is very casual and can sound impolite or rude to some. For example: (from formal to casual) Hello, how do you do? Hello, how are you? Hi how are you? Hey what's up!

It's a problem of relationship between you and someone else. In my experience, people use 'hello'(the formal form) to ones who they meet at first or are not in quite familiar with. Hi and hey is use in situation when you meet someone friendly.

I would say that "hello" and "hi" are interchangeable. They are both perfectly fine for greeting people in a wide variety of situations, although some people consider "hello" to be more formal than "hi". "Hey" can also be used to greet someone, but it's much more informal, and usually only used between friends. It can also be used to get someone's attention.