Wiring 3 proximity sensors in series
You might be able to make this work but there are some sticky issues. First, if the ground connections of the sensors are not connected to the same ground as the controller then the sensors will not be able to pull their outputs down to the controller ground. As a result the controller will never see a logic low level from the sensor. One way to correct for this is to add an optoisolator at the output of every sensor. The output of the sensor would then activate an LED in the optoisolator and the phototransistor in the optoisolator could activate the controller input. This is pretty easy and inexpensive.

The harder problem is providing a stable supply voltage to the three sensors. When circuit elements are connected in series we know that the current through them must be the same but the voltage across them can be different. You need a way to make sure that the voltage across each sensor remains relatively constant even though the current they draw varies over time. I'm thinking that it may be enough to put a 16V, 1W, zener diode and a sizable capacitor in parallel with each sensor's power connections.

If you're not comfortable doing this then I would suggest just creating a 12V or 15V power supply using a linear regulator (say a 7812) from the 48V supply. You only need about 30mA to operate the three sensors, and I can't think of any problem that would occur by doing this.