3mm refers to the diameter of the incoming filament that goes into the extruder. Inside the extruder Hot End that filament melts, then is pushed out of the smaller diameter hole on the bottom of the hot end. The nozzle size of the hot end is what determines how high a resolution you can print. For example, a 0.50mm nozzle can print a line 0,50mm wide, but of varying thicknesses depending on the settings you choose. You can actually get very fine detail with a larger nozzle if you are willing to use very thin layers, it just takes a long time. The best medium quality vs. time it takes to print nozzle balance is usually the 0.35mm nozzle.
1.75mm filament is the other major diameter of filament. It’s much smaller, which has some advantages for different types of extruders. The main reason you want to pay attention to that measurement is to make sure you buy the correct material type for your printer.