Hi
I've done quit a few irpicktool applications but I always use the encoder (never the sensor) but I give some points.
You said the robots are independent and want to split the load. Most of the time (if not always) you split the load when you have a master and slaves. The communication is through RIPE. Not your case.
I think you should have three sensors but again not your case.
I need to know first how do you connect one sensor to the 3 robots. Is it the same input and you send it to the 3 robots ? If you disconnect robot 1, is robot 2 doing the right job ?