When the ph of a solution is changed from 3 to 4 they hydronium ion concentration of the solution

  The pH scale is a logarithmic scale. Each change of 1 unit in pH represents a 10-fold change in hydrogen ion concentration. As the pH gets smaller the hydrogen ion concentration gets larger. Since you have gone from pH 5 to pH 3 there is a 100-fold increase in [H+]. 

For pH = 5; [H+] = antilog 10^-pH = 1.0 x 10^-5 M 
For pH = 3; [H+] = antilog 10^-pH = 1.0 x 10^-3 M 

(1.0 x 10^-3)/(1.0 x 10^-5) = 1.0 x 10^2 hope this helps

The effective strength of an acid or base in solution, or acidity, will vary with both the intrinsic strength of the acid or base itself and the amount of the acid or base which is present in the aqueous solution.