While I technically agree with Liam's answer, I think there is two reasons why I would act differently: Purely for psychological reasons (personal peace of mind) and to keep redundancy consistent.
First let me constrain the scenario: I assume you haven't made a major mistake. One piece is out, so one could argue you made some kind of mistake - but that's ok, mistakes happen, having a system to catch them is the important thing, and that's what we are discussing. A major mistake would e.g. be using the same rock feature for two points and the one point failed due to that feature being loose (or loosened by the leverage of the camming device). In short: Lets assume there is no reason to doubt the two remaining points.
Now there also wasn't any reason to doubt the point that failed - still it did. So I'd try to add another point, to have a three-fold redundancy again. Now I know that argument is probably flawed if you consider the likeliness of two simultaneous failures. On the other hand there's also no reason to believe the failures are necessarily independent. Maybe it's a different type of rock than you are used to, you have a bad day (dismissing that as "you must not be climbing in that case" as often done is dangerous, this is not a binary condition), ... - essentially you just missed something that also applies to the remaining two points. In that spirit you would now adjust (lower) your trust level to those anchors and add another one.
None of these are any "hard arguments", it's very much up to the specific situation and as soon as you are at a safe point again, try to evaluate that situation and your reaction to it. Not to blame yourself, but to learn and maybe find something you can improve when you encounter this situation again.
And let me make a tangential point that will probably be a bit annoying due to being pedantic and knowingly misinterpreting words (because I am sure someone will misinterpret them), but I feel rather strongly about it, so I am doing it anyway:
You should always be worried about all your anchors failing. Even the best placed anchor can fail. There is no such think as safety, you are always managing a risk.
Now this has no immediate practically applicable implication, but I believe it's a very important mindset to reduce and accept risk successfully. Because if you are belaying two seconds on self-placed gear and are not aware, that there is a risk (maybe a slim one, but still), you are way more likely to make a mistake.
Most of this is just an opinion piece which is purely backed by having seen a few mistakes. And then when addressing the involved party, always in a neutral manner inquiring about why they did it this way (maybe there is a reason I am ignorant of - another mindset I like :) ), I got dismissed with some excuse along the line of a general "I am doing it safe, no need to think about it", which is plain dumb.