Before you worry about whether you should export as P3, does your camera even export in P3? Most cameras today still only export in either sRGB or Adobe RGB.

However, if you're using an iPhone for capture (or any other camera that supports P3), then your images support P3. When a P3 image is rendered on an sRGB screen, software that works correctly will find the nearest renderable color for each pixel and display that. Is that good enough for your uses? Software that doesn't render correctly will simply interpret the values as sRGB and it will look different (maybe good, maybe bad, depending on what's in it and your personal tastes).

If you're thinking mainly about web pages, there are techniques you can use to detect the browser in use, and information about the display. (I'm not sure whether you can detect whether a display supports P3 or not from, say, Javascript.) You could show sRGB versions to people on browsers you know don't properly deal with color profiles, and show P3 to people on browsers which do handle it or at least handle color profiles correctly. It all depends on how important color fidelity is to your website.

For apps, it should be easier since you're running on the device and can query it directly. Your individual use case will determine whether it's important or not. I work on software where we have determined that it is important, and we now support P3. Know this - it is not easy to get correct and your users will likely misuse such support. Having good documentation and defaulting it to off is probably your best bet at the moment. But it is certainly possible and software is starting to support it. Now is a good time to at least start thinking about it.