The given sequence of reaction is 

$\mathrm{A}\stackrel{{\mathrm{k}}_1}{⟶}\mathrm{X}\stackrel{{\mathrm{k}}_2}{⟶}\mathrm{Y}\stackrel{{\mathrm{k}}_3}{⟶}\mathrm{Z}$

where, ${\mathrm{k}}_3>{\mathrm{k}}_2>{\mathrm{k}}_1$

Now, we know that the lower the value of rate constant indicates that the reactants will take more to get convert into products, therefore ${\mathrm{k}}_1$ has lowest value so $\mathrm{A}$ to $\mathrm{X}$ conversion is the slowest step in the given sequence and hence it will be rate determining step.