PhD applications: ethics of not informing other universities after accepting an offer

I think you should withdraw your applications from B and C.

By keeping your application active, not only are you wasting the committee's time, you are potentially hurting other students. Most graduate programs can only accept a certain number of students per year, but there may be many more applicants who would still be well qualified and able to succeed. If the committee accepts your application, that means they are denying admission to someone else who might be equally deserving. If you withdraw now, you open that space for someone else.

If your only purpose is to just to see if you get in (presumably because getting into a prestigious university feels good and is rewarding, even if you ultimately attend a different prestigious university), then I would say that it's very ethically questionable, and recommend that you withdraw your application.

As mentioned above, if they decide to accept you, that typically results in another student being rejected or waitlisted. Even if you paid the application fee, you choosing not to withdraw is not simply "wasting their time" in return for your fee, it's wasting their time while also hurting another student for whom this university may be their first and best choice.

If you want to attend university A, then politely withdraw your application from B and C. If you aren't 100% sure that you want to attend university A, then it's perfectly acceptable to see if B or C accepts you, and then figure out which one is the best fit for you, but it doesn't sound like that's your intent.

I am sympathetic to your desire to get some feedback on your attractiveness as an applicant at prestigious universities. In the context of this debate, it's worth keeping in mind that in academia, getting formal feedback of this sort -- answers to graduate applications, and later paper submissions, grant proposals, job applications, etc. -- is really crucial to signal to us about the quality of the work we are doing, which in turn is helpful for us to later make actual decisions that influence the future course of our careers. Moreover, this sort of feedback can be quite infrequent and hard to come by, so your desire to take advantage of a potentially rare opportunity to receive it, even by employing a course of action that may be seen as dishonest in a small way, is understandable, even reasonable: if a small dishonest and essentially harmless act helps you fulfill your potential in life later on, that seems like a net benefit not just to you but also to the rest of the world.

With that said, I would still encourage you to inform universities B and C that you no longer wish for your applications to them to be considered. It's not really about the ethics issue, which I see as impossible to answer with any degree of consensus -- some people will think it's ethical because you paid the application fees and are therefore entitled to get the service you paid for in the form of an answer to your applications; others will argue that you are thinking of using the admissions system in a way that it was clearly not meant to be used, and causing a needless waste of resources. I see this discussion as relatively unimportant, because ultimately the stakes are very low: yes, you may waste 10 minutes of someone's time, but if you promptly inform universities B and C that you are declining their offer if and when they end up accepting you, there will be no harm done to anyone. So it's an abstract discussion with no real practical significance -- fun to talk about perhaps, but in itself a waste of time.

The real reason I think you should withdraw is that the information you will get by waiting to hear an answer is actually not very useful at all, and potentially even misleading. Mostly it will give you a sense that you have learned something about yourself and how the world views you, and satisfy some psychological need that we all have for knowing what people think of us, but that feeling will actually be illusory. The point is that admissions decisions depend on so many factors, most of which you have no knowledge of, that they have a very low signal-to-noise ratio. Moreover, in a situation like yours, where you have already been admitted to your top choice (and very prestigious) school, which is already very good positive feedback, the knowledge that you were also accepted to schools B and/or C, or rejected from them, would be worth even less. Again to use a math/engineering terminology (hope that makes sense to you), you can think of this as a situation where you are about to receive two bits of information that are essentially almost completely random, but the bits are labeled in a way that makes them seem like they carry real, useful information. So in my opinion the psychological satisfaction you will get from being told these bits is actually outweighed by the risk that you will incorrectly attribute too much real significance to the meaning the bits supposedly carry. That's what I meant when I said above that the answers from B and C could even be potentially misleading. Ultimately, these answers are simply not useful information.

Hope this helps, and congratulations for getting into your preferred school!