Small AC-DC conversion/amplification
I think the premise that the output is 3V AC is incorrect. But looking at the datasheet i can see where the assumption is coming from:

Source: Datasheet
Notice the output configuration is a three-wire connection with a SPST solid-state ac switch. This might be easier to understand looking at the wiring diagram, also provided in the datasheet:

Wire 1 and 3 are for the supply voltage of 20V-250V ac. The output signal of the sensor is basically the state of the switch either being ON or OFF. This output can be used with wire 4 connected to the load or in other words, your circuit.
The most important rating is the maximum output current of 300mA. I believe the &quot;ON-state saturation voltage&quot; of 3V that is mentioned at 300mA is just the voltage drop across the switch.
So in practice, the output voltage is approximately equal to the supply voltage in ON-state. Depending on the supply voltage, you could just use a regular rectifier for your PC input then.