2 and 3 both sound OK to a British English speaker. On "buy", in 2, the focus is on the current situation - the speaker sees the dress and so the "buying" has a repercussion in the present moment even though the buying clearly took place before the present. However, in 3, the focus is a bit more on the past. If in doubt, it is safer to use the past simple when referring to a single event in the past. On "get/got", the first thing to know is that "have got" usually has the present tense meaning of "have". e.g. I have a car = I have got a car. In UK English, "have got" is also the present perfect of "get" but in US English, it is "have gotten". Still on "get", only the past simple works here in most contexts if we imagine that the wearer bought it in a shop.

Hi there! I think that all of them are in the past tense (bought).

I dislike the word "get", so I will use "buy" instead. However, the analysis is the same. "Where did you buy it" and "where have you bought it" are, grammatically speaking, equally correct. Since you are asking for a simple fact, an event that occurred at one instant, there is no reason to say "have bought". The present perfect tense is more difficult to understand and has a variety of interpretations. Unless there is a need for it, just stick with the simple past. It is honest and direct, says exactly what it says, and does not need to be interpreted.