  Moir\'e superlattices have recently emerged as a novel platform where
correlated physics and superconductivity can be studied with unprecedented
tunability. Although correlated effects have been observed in several other
moir\'e systems, magic-angle twisted bilayer graphene (MATBG) remains the only
one where robust superconductivity has been reproducibly measured. Here we
realize a new moir\'e superconductor, mirror symmetric magic-angle twisted
trilayer graphene (MATTG) with dramatically richer tunability in electronic
structure and superconducting properties. Hall effect and quantum oscillations
measurements as a function of density and electric field allow us to determine
the system's tunable phase boundaries in the normal state. Zero magnetic field
resistivity measurements then reveal that the existence of superconductivity is
intimately connected to the broken symmetry phase emerging from two carriers
per moir\'e unit cell. Strikingly, we find that the superconducting phase gets
suppressed and bounded at the van Hove singularities (vHs) partially
surrounding the broken-symmetry phase, which is difficult to reconcile with
weak-coupling BCS theory. Moreover, the extensive in situ tunability of our
system allows us to achieve the ultra-strong coupling regime, characterized by
a Ginzburg-Landau coherence length reaching the average inter-particle distance
and very large $T_\mathrm{BKT}/T_{F}$ ratios in excess of 0.1, where
$T_\mathrm{BKT}$ and $T_F$ are the Berezinskii-Kosterlitz-Thouless transition
and Fermi temperatures, respectively. These observations suggest that MATTG can
be electrically tuned close to the two-dimensional BCS-BEC crossover. Our
results establish a new generation of tunable moir\'e superconductors with the
potential to revolutionize our fundamental understanding and the applications
of strong coupling superconductivity.
