x= the coefficients in front of the substance in the balanced chemical equation
[H+]= the concentration of hydrogen ions
[A-]= the concentration of the other ion that broke off from the H+
[HA]= the un-disassociated acid concentration
The higher the Ka value, the greater amount of disassociation of the reactants into products. As for acids, they will break down to form H+ ions. The more the H+ ions, the stronger acidity of the solution. Thus since A has the highest Ka value, that represents the strongest acid.
You can determine the Ka value from a number of ways. If equilibrium concentrations are given of a certain acid solution, you can find the proportion of the concentration of ions to the concentration of the remaining HA molecules, using the equation above. Also, pH and KpH can be used in a number of ways. This gets more complicated and depends on the situation, and requires more advanced equations.
Hope this helped a little, its obviously not my best work