Question

A closed flask contains a substance in all its three states

Answer 1

In thermodynamics, the triple point of a substance is the temperature and pressure at which the three phases (gas, liquid, and solid) of that substance coexist in thermodynamic equilibrium.[1] For example, the triple point of mercury occurs at a temperature of −38.83440 °C and a pressure of 0.2 mPa.

In addition to the triple point for solid, liquid, and gas phases, a triple point may involve more than one solid phase, for substances with multiple polymorphs. Helium-4 is a special case that presents a triple point involving two different fluid phases (lambda point).[1]

The triple point of water is used to define the kelvin, the base unit of thermodynamic temperature in the International System of Units (SI).[2] The value of the triple point of water is fixed by definition, rather than measured. The triple points of several substances are used to define points in the ITS-90 international temperature scale, ranging from the triple point of hydrogen (13.8033 K) to the triple point of water (273.16 K, 0.01 °C, or 32.018 °F).

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Answer 2

there are three states of matter that is the solid, liquid and gases, all these three elements of matter are present in a closed flask, the gaseous part is in the empty space in a closed flask,  while the liquid state is the vapor cooled and condensed back to liquid, the solid state is the physical flask itself. the flask has also a vacuum which does not contain any state of matter.