  We construct a van der Waals heterostructure consisting of three graphene
layers stacked with alternating twisting angles $\pm\theta$. At the average
twist angle $\theta\sim 1.56^{\circ}$, a theoretically predicted magic angle
for the formation of flat electron bands, narrow conduction and valence moir\'e
bands appear together with a linearly dispersing Dirac band. Upon doping the
half-filled moir\'e valence band with holes, or the half-filled moir\'e
conduction band with electrons, displacement field tunable superconductivity
emerges, reaching a maximum critical temperature of 2.1 K at optimal doping and
displacement field. By tuning the doping level and displacement fields, we find
that superconducting regimes occur in conjunction with flavour polarization of
moir\'e bands bounded by a van Hove singularity (vHS) at high displacement
fields. This experimental observation is found to be inconsistent with a weak
coupling description, suggesting that the observed moir\'e superconductivity
has an unconventional nature.
