No you can't 'share' one resistor for the 3 isolators.
However you can use no resistors if you just enable the internal pull-up resistors for each input pin you use.
do it like this for each input pin used:
pinMode(PinX, INPUT); // set pin to input mode
digitalWrite(PinX, HIGH); // enables the internal pull-up resistor
That will provide the collector current for the optoisolator's output transistor. A digitalRead of LOW means the opto is turned on and a HIGH reading means the opto is off.
Lefty