Digital meters can be autoranging or manual ranged (knob controlled). Old analog meters were all manual ranged. the "range" specifies the maximum value that can be displayed. for example, you may have a 2 volt scale, a 20 volt scale and a 200 volt scale on a unit that can display 199 as its max value. On the 200 volts scale (technically 199) you can only see whole volts. On the 20 volt scale you would see 19.9. On the 2 volt scale you would see 1.99. Therefore the 2 volt scale is needed for reading 1.5 volt batteries with any accuracy. An autoranging meter puts the decimal in the right place for you automatically so you don't have the adjust the knob as you go from one voltage measurement to another. Generally autoranging is considered nicer/better, but some of them are slower to read and settle on a number, so some folks like the speed of a manual ranger.

if you say it only has 3 ranges, then it probably really has 3 functions, AC volts, DC volts and ohms and is autoranging.