  We derive three parameters associated with the angular distribution of
semileptonic five-body decays $D_{(s)}/B_{(s)}\to
V\ell^+\nu_{\ell}\,(\ell=e,\mu,\tau)$ with $V\to PPP$, where $V$ and $P$ denote
vector and pseudo-scalar particles. These parameters, expected to be unity in
the Standard Model, may deviate if new physics is involved. Our
model-independent approach involves deriving the specific form of the angular
distribution under the most general form of the decay matrix element
$D_{(s)}/B_{(s)}\to V$. The outcomes have potential applications in precisely
testing the Standard Model and exploring new physics. Relevant measurements can
be carried out using data obtained from BESIII, Belle~II, and LHCb.
