You can use a Bluetooth® Smart compatible heart rate sensor, such as the Suunto Smart Sensor, with your Suunto 3 Fitness to get accurate information about your exercise intensity.
If you use the Suunto Smart Sensor, you also have the added advantage of heart rate memory. The sensor memory function buffers data if the connection to your watch is interrupted, for example, when swimming (no transmission under water).
It also means you can even leave your watch behind after starting a recording. For more information, please refer to the Suunto Smart Sensor User Guide.
See Pairing PODs and sensors for instructions on how to pair an HR sensor with your watch.