  The observable $\mathcal{R} ( D^{(*)} ) = \mathcal{B}\left( B^{0}\to D^{(*)-}
\tau^{+} \nu_{\tau} \right) / \mathcal{B}\left( B^{0}\to D^{(*)-} \mu^{+}
\nu_{\mu} \right)$ is a probe for Lepton Universality violation, so it is
sensitive to New Physics processes. The current combination of the measurements
of $\mathcal{R} ( D^{(*)} )$ differs from Standard Model predictions with a
$4\sigma$ significance. A measurement of $\mathcal{R} ( D^* )$ using
three-prong $\tau$ decays is currently ongoing at LHCb. The statistical
precision of this analysis is 6.7%, i.e. the smallest statistical uncertainty
for a single measurement of this observable. Therefore this measurement will be
important to confirm or disprove the current discrepancy from the theoretical
expectations.
