Quantum geometry of electronic wavefunctions results in fascinating
topological phenomena. A prominent example is the intrinsic anomalous Hall
effect (AHE) in which a Hall voltage arises in the absence of an applied
magnetic field. The AHE requires a coexistence of Berry curvature and
spontaneous time-reversal symmetry breaking. These conditions can be realized
in two-dimensional moir\'e systems with broken xy-inversion symmetry
(C2z​) that host flat electronic bands. Here, we explore helical trilayer
graphene (HTG), three graphene layers twisted sequentially by the same angle
forming two misoriented moir\'e patterns. Although HTG is globally
C2z​-symmetric, surprisingly we observe clear signatures of topological
bands. At a magic angle θm​≈1.8∘, we uncover a
robust phase diagram of correlated and magnetic states using magnetotransport
measurements. Lattice relaxation leads to large periodic domains in which
C2z​ is broken on the moir\'e scale. Each domain harbors flat topological
bands with valley-contrasting Chern numbers ±(1,−2). We find correlated
states at integer electron fillings per moir\'e unit cell ν=1,2,3 and
fractional fillings 2/3,7/2 with the AHE arising at ν=1,3 and 2/3,7/2.
At ν=1, a time-reversal symmetric phase appears beyond a critical electric
displacement field, indicating a topological phase transition. Finally,
hysteresis upon sweeping ν points to first-order phase transitions across a
spatial mosaic of Chern domains separated by a network of topological gapless
edge states. We establish HTG as an important platform that realizes ideal
conditions for exploring strongly interacting topological phases and, due to
its emergent moir\'e-scale symmetries, demonstrates a novel way to engineer
topology