A bridgeless graph $G$ is called $3$-flow-critical if it does not admit a nowhere-zero $3$-flow, but $G/e$ has for any $ein E(G)$. Tuttes $3$-flow conjecture can be equivalently stated as that every $3$-flow-critical graph contains a vertex of degree three. In this paper, we study the structure and extreme edge density of $3$-flow-critical graphs. We apply structure properties to obtain lower and upper bounds on the density of $3$-flow-critical graphs, that is, for any $3$-flow-critical graph $G$ on $n$ vertices, $$frac{8n-2}{5}le |E(G)|le 4n-10,$$ where each equality holds if and only if $G$ is $K_4$. We conjecture that every $3$-flow-critical graph on $nge 7$ vertices has at most $3n-8$ edges, which would be tight if true. For planar graphs, the best possible density upper bound of $3$-flow-critical graphs on $n$ vertices is $frac{5n-8}{2}$, known from a result of Kostochka and Yancey (JCTB 2014) on vertex coloring $4$-critical graphs by duality.
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