I didn’t see anywhere on the page that explains the color coding of the wires, so I can’t say for sure, but generally speaking, I assume red is 5V, black is GND, and yellow is signal (SN1).
Be careful that these types of flow sensors generate high pulse rate, and they may not work with OpenSprinkler 3.0 well — the technical reason is that due to some interrupt issues with ESP8266, the flow sensor reading is implemented by a method that combines interrupt and polling, so when the pulse rate is very high (like more than 100 per second), the controller may not be able to catch up with the fast pulse and so the flow reading can be inaccurate.