You CAN....
Connect 3 DS18B20s to an Arduino by using three pins, and a ground connection.
You SHOULD... if not terribly difficult... use non-parasitic powering for the system....
However... if you don't mind working (much) harder on the software side, put 3 (or more) DS18B20s on a single pin of the Arduino. This means just 3 wires, instead of 5....
1-Wire data
Power
Ground
You then get into the 1-Wire techniques of chip addressing. The three devices remain "quiet" until you send a "tell me the temperature" command out on the 1-Wire data line... but you have to say WHICH chip you are sending that command to.
The simpler technique, using 5 wires....
Power
Ground
and a separate 1-Wire data wire to each chip
... works in a similar way, but as there is only one temperature sensor on each data wire, you can skip the addressing part.
(Much) more on 1-Wire at...