You’re going to need to connect the sensors one at a time to the Arduino board in order to program the address. Then you’ll need to hook them all together to the same bus. For example, all 6 of the sensor power wires need to go to the Vcc power bus, all 6 of the sensor ground wires need to go to the board GROUND, and all 6 of the sensor data lines need to go to a data pin (I like to use D7 for my SDI-12 data lines because it’s rarely used by other things like accessory shields).
So your challenge is going to be getting 3 sets of 6 wires all connected to just 3 pins on the Arduino board. This can be accomplished in a variety of ways, depending on how comfortable you are with soldering or stripping wires or making a custom board with some screw terminals. Or you might be able to do it with a breadboard if you don’t mind cramming the sensor wires into the holes (it’s generally bad practice to cram medium-gage bare wires into a solderless breadboard because it permanently widens the contacts making it unreliable for later use with smaller components).
Do you have a breadboard, or some spare screw terminals, or a screw-terminal breakout shield for the Duemilanove? Or maybe you just want to use a wire nut to join each of the 6 wires together along with one wire to connect to Arduino board? It’s usually not recommended to join that many wires with one wire nut, but it might be possible if you do it just right. Is this just for a temporary test/experiment, or are you thinking of a long term setup?
Once we figure out the wiring, I can post the sample code for cycling through 6 channels. How often do you want to take a sample, and is it just being displayed on the computer using the Arduino IDE’s serial monitor? If you used a Mayfly the data could also be time-stamped and stored on the memory card.