Our TT also has 3 valves. Two are fresh water drain valves and one is the low point valve/drain.
The operation is described in our manual under -DRAINING THE FRESH WATER SYSTEM
You can "open the low point drain by turning, then pulling the handles up".
The fresh water valves drain the fresh water from your plumbing and the low point drain will allow that water to drain to the ground outside of your rig.
Richard & Marilyn
2010 Ford F150, V8, 5.4 liter
2010 Jay Feather Sport 165