We have seen that we could have an equilibrium between water as a liquid and a solid by keeping the temperature at 0°C. What would you have to do to also have equilibrium with water as a gas?
All you have to do is cover the glass. Once it is sealed, water will evaporate until it reaches equilibrium. At that point, the number of water molecules moving from the air to the liquid will balance the number of water molecules moving from the water to the air. if the temperature is still kept at 0°C, all three of the states of water will remain balanced.