We prove an optimal Kawamata-Miyaoka type inequality for terminal $\mathbb Q$-Fano threefolds with Fano index at least $3$. As an application, we obtain that any terminal $\mathbb Q$-Fano threefold $X$ satisfies the following Kawamata-Miyaoka type inequality \[ c_1(X)^3 < 3c_2(X)c_1(X). \]