On what date were you told you were 3rd on the list? I can’t really follow your timeline.
Unfortunately, I personally can’t see that anything has gone wrong with the process, but I do think it was an error for whomever it was at UNSW who gave you the waitlist information to have done so. It is a very common occurrence that people within Medicine admissions do not have all the information and tend to be either very rigid with giving no information, or too eager to give the desperate person on the other end of the phone something hopeful that they say too much (and often inaccurate) information.
I called UTAS last year at one point and was told by the person on the other end of the line, who said they logged into my application, that I would “definitely have an offer by the end of the week”. Not only did I never get an offer, but I ended up being 6 UMAT raw marks off even being competitive.
I know it’s frustrating and distressing, but it’s also a common occurrence that incorrect and falsely hopeful information is provided by admissions staff who are just trying to help.