I'm new to the site, and I just un-winterized our new Sunseeker 3170 this weekend for our first trip. The 3 valves are the water heater bypass kit. Close the middle valve and open both the top and bottom valves. Cold water flows in the bottom & hot flows from the top, these pipes are typically colored blue and red.
The middle valve connects the hot-to-cold, this allows you to bypass the water heater for winter storage so the water heater can be drained.