The valves in the picture I posted are regular "turn till open" or "turn till closed" valves. Some trailers have the same valve configuration but have 1/4 turn valves where the handle pointed across the pipe direction is closed and the valve handle pointed parallel to the pipe is open. Same results either way.
There are still other winterization valve arrangement involving only two valves but you indicated you had a three valve system which would be like the picture.
By the way, I think my water heater plug takes a 15/16" socket (Atwood heater). I think the Suburban heater takes a 1".
Barney