3 is currently, according to the wiki, used as an artist tag for an artist named 3.
But looking over the images tagged with it, it appears to be getting used primarily for any image with the numeral 3 in it. Fortunately, it's not used for many images, so it could be easily fixed either way.
Should it be split into 3 and 3_(artist) or some such thing?
Updated by henmere