You'll need to instantiate 3 separate instances of the one-wire object.
Untested, just duplicating lines inside my working 1-wire sketch, and adding alpha suffixes to the variable names.
// Data wires plugged into digital pins 2,3,4 on the Arduino
#define ONE_WIRE_BUSA 2
#define ONE_WIRE_BUSB 3
#define ONE_WIRE_BUSC 4
// Setup a oneWire instance to communicate with any OneWire devices (not just Maxim/Dallas temperature ICs)
// Pass our oneWire reference to Dallas Temperature.
Then you can read your sensors using the usual functions, e.g.
Of course, your code will be much neater if you define arrays for these. Hmmm. I'm not sure how the declaration would look for that. You'd want the equivalent of
Except int would be the DallasTemperature type, and I don't know about doing that without the one-wire reference. I'm an okay C programmer, C++ not so much.