That's a very simple (and possibly error prone) multiplexer. Here's what I think they did: they connect the three touch sensors with a bunch of resistors to the analog signal wire. Basically, you have a variable voltage divider. Depending on which sensors are pressed, the voltage level will be different. Now this may be error prone, as the AD port may very well return intermediate values and a driver should make sure that 2 or 3 consecutive readings were in the same voltage interval.
However, on the mindsensors homepage the compatibility icons say, that this sensor does NOT work with the EV3.
Also, it certainly has been designed and tested with NXT touch sensors only.
Also see http://www.mindsensors.com/index.php?mo ... AGE_id=135
I'm not sure, why they say that the multiplexer is not compatible. Is it the lack of drivers? Is it the autodetection mechanism of the EV3? Is it an incompatibility with the new EV3 touch sensors? Does the ADC of the EV3 return values different from the values return by the ADC of the NXT? Who knows.