It does also depend on the full sentence in which they are being used sometimes but the two below are the most natural to use: + The meeting is on "September the 3rd" + The meeting is on "THE 3rd of September" --> please note you need to add "the" to make it natural. The other ones you wrote (3 September / 3 of September / 3 Sep) are not natural here or ever. Here are a couple of other possible alternative, again depending on context though: + The meeting is on "September 3" + The meeting is on "03/09/19" --> Global date format + The meeting is on "09/03/19" --> American date format