  Wigner crystals are predicted as the crystallization of the dilute electron
gas moving in a uniform background when the electron-electron Coulomb energy
dominates the kinetic energy. The Wigner crystal has previously been observed
in the ultraclean two-dimensional electron gas (2DEG) present on the surface of
liquid helium and in semiconductor quantum wells at high magnetic field. More
recently, Wigner crystals have also been reported in WS2/WSe2 moir\'e
heterostructures. ABC-stacked trilayer graphene on boron nitride (ABC-TLG/hBN)
moir\'e superlattices provide a unique tunable platform to explore Wigner
crystal states where the electron correlation can be controlled by electric and
magnetic field. Here we report the observation of magnetic field stabilized
Wigner crystal states in a ABC-TLG/hBN moir\'e superlattice. We show that
correlated insulating states emerge at multiple fractional and integer fillings
corresponding to v = 1/3, 2/3, 1, 4/3, 5/3 and 2 electrons per moir\'e lattice
site under a magnetic field. These correlated insulating states can be
attributed to generalized Mott states for the integer fillings (v = 1, 2) and
generalized Wigner crystal states for the fractional fillings (v = 1/3, 2/3,
4/3, 5/3). The generalized Wigner crystal states are stabilized by a vertical
magnetic field, and they are strongest at one magnetic flux quantum per three
moir\'e superlattices. The correlated insulating states at v = 2 persists up to
30 Tesla, which can be described by a Mott-Hofstadter transition at high
magnetic field. The tunable Mott and Wigner crystal states in the ABC-TLG/hBN
highlight the opportunities to discover new correlated quantum phases due to
the interplay between the magnetic field and moir\'e flatbands.
