I haven’t got my RPi 3 yet. The spec seems to suggest that RPi 3 draws significantly more current. I can’t say whether it will work on existing OSPi or not. If not, it should be easy to fix by soldering a new fuse (or solder a wire across the fuse to bypass it). Specifically, OSPi has an on-board 1.1A fuse on the 5V line. The DC-DC converter can output more than 1.5A, but the fuse is capping the output current. So bypassing the fuse will allow the DC-DC converter to supply more current.
To be honest, in my mind the best RPi to work with OSPi is either RPi A+ or Zero, as they are the lowest cost versions, and to run a sprinkler controller you don’t really need the computation power of RPi 2 or 3.