CR123 is a Lithium battery with a nominal value of 3.7 volts.  If you were to measure the voltage it would be between 3 and 4.  Being a battery the voltage will change as the battery is used.  (It can even change based on temperature, age and many other factors.)

There are tons of different names and numbers for the same battery… DL123a, CR123, EL123, K123 are just some examples.

http://www.batteriesdirect.com/product/details/photo-lithium-batteries/energizer/cr123