Hey, cool question!
In the science world (which I am definitely not a part of), this is called the triple point of water, because, well, the three states of water all exist together at this temperature! The technical term for when they coexist is thermodynamic equilibrium. So, you could say that water's triple point in which the three phases of water coexist in thermodynamic equilibrium is 0.01°C.
I found a pretty cool video of a guy showing you how to create a triple point of water cell - they're used for calibrating precision thermometers:
