Perfect Indistinguishability in shift cipher
I'm assuming that, that attacker produce $m_0$ and $m_1$ and given one of their ciphertext as a challenge.


given $c$, the attacker cannot distinguish whether it is encryption of $m_1$ or $m_2$. Since, he doesn't know the key. He can guess only 1/2 probability. For this attacker, it has perfect indistinguishability.
given $c$, the attacker can distinguish that the plaintext has all characters different. Because the shift cipher's property, $$c = E_k(m_1) \text{ and } c=E_k(m_1) \Leftrightarrow  m_1 = m_2$$ and he output $m_0$ and $m_1$ with all different characters.
given $c$, the attacker can distinguish that the plaintext has all characters different. Because the shift cipher's property and and he was output $m_0$ as $aaa$ and $m_1$ as all different characters.
given $c$, the attacker can distinguish that because the attacker output $m_0$ and $m_1$, on advance he knows that first letter is always an $a$


As a result; 2,3, and 4 can be used to demonstrate that the shift cipher for 3-character messages does not satisfy perfect indistinguishability