Actually in British English they are all OK and all still quite often used in formal letters. Further to ... .... is a little bit old fashioned but I still see it used quite often in business letters, legal documents etc. With reference to ... .... this is used quite frequently here in the UK. In regard to ... ... again quite common - but from what the two answers above say it might be more frequently used in the USA. I guess you should chose depending on who your target reader is. If you are writing to someone in the USA then use the third one. For someone in the UK, all of them would be understood and thought to be a "normal but formal" way of phrasing things. Hope that helps

I agree that the third option is the best. The second one is OK, but a little awkward. The first one is not good at all.

In regards to our meeting held on August 20...

I have seen all these used in business correspondence in the US. There is a subtle difference in that using "further to" implies something additional to the topic of the meeting while the other two phrases might introduce a new topic or something related to the circumstances of the meeting itself, rather than the subject matter. A more informal usage would be, "following up on our meeting..."