Three-torsion points are flexes of a plane cubic ($3(P-O)=0$)

Let $E$ be a smooth plane cubic with an inflection point $O$ as identity. A point $P$ with $3(P-O)=0$ in Picard group has a line whose intersection divisor with $E$ is $3P$. Indeed, ${\mathcal{O}}_E(3P)\cong {\mathcal{O}}_E(3O)\cong {\mathcal{O}}_E(1)$. On this genus-one curve, the Riemann-Roch theorem gives $h^0({\mathcal{O}}_E(1))=3$: the dual term has negative degree. Restriction of ambient linear forms is injective and its source also has dimension three, so it is an isomorphism. The section vanishing at $3P$ therefore comes from a line. Conversely, such a line gives $3P\sim 3O$, so $P$ is a three-torsion point. This includes $O$; points of exact order three are the other such points.