  We show, using semiclassical measures and unstable derivatives, that a smooth
vector field $X$ generating a contact Anosov flow on a $3$-dimensional manifold
$\mathcal{M}$ has only finitely many Ruelle resonances in the vertical strips
$\{ s\in \mathbb{C}\ |\ {\rm Re}(s)\in
[-\nu_{\min}+\epsilon,-\frac{1}{2}\nu_{\max}-\epsilon]\cup
[-\frac{1}{2}\nu_{\min}+\epsilon,0]\}$ for all $\epsilon>0$, where
$0<\nu_{\min}\leq \nu_{\max}$ are the minimal and maximal expansion rates of
the flow (the first strip only makes sense if $\nu_{\min}>\nu_{\max}/2$). We
also show polynomial bounds in $s$ for the resolvent $(-X-s)^{-1}$ as $|{\rm
Im}(s)|\to \infty$ in Sobolev spaces, and obtain similar results for cases with
a potential. This is a short proof of a particular case of the results by
Faure-Tsujii in \cite{FaTs1,FaTs2,FaTs3}, using that $\dim E_u=\dim E_s=1$.
