 Solution

Let $N_m=|\{i:c_i=m\}|$. The label likelihood for the mixture weights is proportional to ${\prod }_{m=1}^3{\pi }_m^{N_m}$. Multiplication by the $\mathrm{Dirichlet}({\alpha }_1,{\alpha }_2,{\alpha }_3)$ density and Dirichlet-multinomial conjugacy givesConditional on the labels, the observations and component means provide no further information about $\pi$.