The supercritical temperature, the maximum attainable temperature of superheating and the critical compressibility factor of caesium, rubidium and potassium are correlated through the generalised van der Waals equation of state. This three-parameter equation differs from the known van der Waals equation of state by the modified expression for molecular pressure. For caesium, rubidium and potassium, the ratios of the supercritical temperature to the maximum attainable temperature of superheating are estimated. Our estimation results are in agreement with experimental data. It has been established that caesium, rubidium and potassium obey the single-parameter law of corresponding states with the ratio of the supercritical temperature to the maximum attainable temperature of superheating as the thermodynamic similarity parameter.