The short answer is yes. “3mm” fillament covers a range of fillament sizes from 2.7mm to 3.2mm. Most manufacturers aim for 2.85mm to be compatable with smaller 3.00mm bores that were common with the oldest 3d printer nozzles. The 3.2mm size is common for ABS plastic welding rods, so some manufacturers (usually the chinese ones with bad quality control) sell that size as 3.00mm because the black plastic is readiliy available. Sometimes less honest manufacturers will also deliberatly size down to 2.7mm, sell it on a 3.3lb roll, and figure that no one is actually going to weigh their fillament. You sometimes find this in no name ebay special fillament, and those are the same people who add filler to their mix.
Generally when printing you want to stick to a reputable fillament supplier. For example Village plastics and Push plastics ABS which I use is usually pretty consistant at 2.85mm. There are some slight differences is various colors. I don’t know if it is because the dies have worn at diffferent rates, or if different colors expand more, but Blue fillament from Push tends to be slightly bigger than black across multiple rolls.
At any rate, the size dowsn’t matter all that much as long as it is in the general range to work properly. you need to make sure you put that exact measurement into your slicing program (cura, etc.), and be aware that on poor quality control (cheap) fillament that value can change throughout the roll. it’s critical for keeping your extrusion rate constant so you aren’t over or under extruding though. Think of fillament diameter like bucket size. if you have a 1 gallon bucket, and you need to fill a 10 gallon tub exactly, it should take you 10 trips. If your bucket was actually 1.1 gallon, by trip number 10, you have an extra gallon spilling over the edge. If you have an extruder that is programmed to think it has 2.85mm fillament in it, and you actually have 3.1mm, you end up with an overextruded blobby part.