They mean more or less the same. How are you? = The most formal version. The next two are less formal. How is it going? (How's it going?) How are you doing? > Thank you in advance / Thanks in advance.

The first one doesn't exist. Are you thinking of "How are you going?". This is an Australian variant, I believe. I wouldn't try saying this to other English speakers, as I don't think that it is used by anyone other than Australians. "How are you?" and "How are you doing?" have the same meaning, but "How are you doing?" is more informal. You wouldn't say this to your boss at work or your teacher at school, for example. Also note that you must write these with a capital letter (H) at the beginning and a question mark at the end (?).