I have data that uses three-digit ZIP code as its geo location data. ArcGIS Online does not seem to recognize ZIP3 as geo-data when I import it, or when I try to join it to an existing ZIP3 layer. I am a fairly new user, so I might be missing something obvious. Is there a simple way to do this in ArcGIS online?
Can you describe your workflow? How are you trying do get the results you are after in ArcGIS Online? What are your desired results?
Here is my take on your issue: you have some data that you want to 'geo-locate' via the 3 digit zip value. However, you are unable to do so. To geo-locate (or geocode as I prefer) a piece of information, you need to match your data to a some geographic data. For example, if you have a list of street addresses in the form of '1234 S Main St' you might want to match that to GIS feature of centerlines that is set up to interpolate the position of that address on a centerline segment.
In all the years I've been geocoding (close to 20 now), I've never heard of the three digit zip code had to google it. What you need is a GIS feature class of those three digit zipcodes to match to.
I just did a search in ArcGIS Online / Living Atlas and only found standard zip codes. I'm not sure where you would find a GIS feature layer of them; you may need to do some digging or create your own.