The ray $\mathrm{PQ}$ will undergo minimum deviation in prism $\mathrm{A}$ if the angle of the incident on $\mathrm{A}$ is such that the refracted ray in $A$ is parallel to its base. Since prisms $\mathrm{B}$ and $\mathrm{C}$ are identical to $\mathrm{A}$ and are made of the same materials as $\mathrm{A}$, the refracted ray in $\mathrm{A}$ suffers no further deviation by prisms $\mathrm{B}$ and $\mathrm{C}$. Hence the emergent ray from prism $\mathrm{C}$ will suffer the same deviation as that produced by prism $\mathrm{A}$. Hence the correct choice is $3$.