  Let $\pi$ be a Hecke cusp form for $\mathrm{SL}_3(\mathbb{Z})$. We bound the
second moment average of $L(s,\pi)$ over a short interval to obtain the
subconvexity estimate $$ L(1/2+it, \pi) \ll_{\pi, \varepsilon}
(1+|t|)^{3/4-1/8+\varepsilon}. $$
