Unfortunately the pre-configured SD card image hasn’t been updated to support RPi 3 yet. So at the moment you need to install a fresh Raspbian and follow what Samer said to install OpenSprinkler firmware.
I know people often want the latest and fastest RPi, but honestly unless if you need the computation power of RPi 3, I would highly recommend sticking with RPi A+ as it’s one of the cheapest options, power efficient, and the computation speed is more than sufficient for a sprinkler controller 🙂