  The resonant profile of the rate coefficient for three-body recombination
into a shallow dimer is investigated for mass-imbalanced systems. In the
low-energy limit, three atoms collide with zero-range interactions, in a regime
where the scattering lengths of the heavy-heavy and the heavy-light subsystems
are positive and negative, respectively. For this physical system, the
adiabatic hyperspherical representation is combined with a fully semi-classical
method and we show that the shallow dimer recombination spectra display an
asymmetric lineshape that originates from the coexistence of Efimov resonances
with St\"uckelberg interference minima. These asymmetric lineshapes are
quantified utilizing the Fano profile formula. In particular, a closed form
expression is derived that describes the width of the corresponding Efimov
resonances and the Fano lineshape asymmetry parameter $q$. The profile of
Efimov resonances exhibits a $q-$reversal effect as the inter- and
intra-species scattering lengths vary. In the case of a diverging asymmetry
parameter, i.e. $|q|\to \infty$, we show that the Efimov resonances possess
zero width and are fully decoupled from the three-body and atom-dimer continua,
and the corresponding Efimov metastable states behave as bound levels.
