In the above substance, the correct order of acidity of positions X, Y and Z is

Carboxylic acid is stronger acid than $-N{H}_3^{+}$ therefore, X is the strongest acid. Since -COOH has −I effect which decreases with distance, therefore, −I effect is more pronounced on Y than on Z. As a result, Y is more acidic than Z. 

Thus, overall order of decreasing acidic strength is X  >  Y >  Z.