  We investigate the ability of the $S_3$ scalar leptoquark to address the
recent hints of lepton universality violation in $B$ meson decays. The $S_3$
leptoquark with quantum numbers $(\overline{\mathbf{3}},\mathbf{3},1/3)$
naturally emerges in the context of an $SU(5)$ GUT model without any conflict
with the stringent limits from observed nucleon stability. Scalar leptoquark
$S_3$ with left-handed couplings to 2nd and 3rd generations of charged leptons
and down-type quarks seems well-suited to address both $R_{K^{(*)}}$ and
$R_{D^{(*)}}$. We quantify this suitability with numerical fits to a plethora
of relevant flavor observables. The proposed $SU(5)$ model calls for a second
leptoquark state, i.e., $\tilde{R}_2$ with quantum numbers
$(\mathbf{3},\mathbf{2},1/6)$, if one is to generate gauge coupling unification
and neutrino mass. We accordingly include it in our study to investigate
$\tilde{R}_2$'s ability to offset adverse effects of $S_3$ and thus improve a
quality of numerical fits. A global fit of the leptoquark Yukawa couplings
shows that large couplings of light $S_3$ to $\tau$ leptons are preferred. We
furthermore identify $B \to K^{(*)} \bar\nu\nu$ as the most sensitive channel
to probe the preferred region of parameter space. Large couplings of $S_3$ to
$\tau$ leptons are finally confronted with the experimental searches for $\tau$
final states at the Large Hadron Collider. These searches comprise a study of
decay products of the leptoquark pair production, as well as, and more
importantly, an analysis of the high-mass $\tau\tau$ final states.
