Most likely this sensor you linked to should work ok with OSPi. Actually most 3-wire flow sensors you can find on the market should be fine, but we want to be cautious because there are such a large variety of different flow sensors that it’s hard to test all of them. The main difference between the 3-wire and 2-wire type is that the 3-wire sensors needs to be powered (typically by 5VDC), and they use hall effect sensors internally. The 2-wire sensors do not need to be powered and internally use a reed switch. To use the 3-wire type, you can power it using the VIN (5VDC) pin from the OSPi or OS board, and connect the ground (GND) wire and data wire to OSPi or OS as the user manual describes (one pin on the sensor port is internally wired to circuit ground, and the sensor’s ground wire should be connected to that pin).