In 1975, Bollobás, Erdős, and Szemerédi asked for the smallest $τ$ such that an $n \times n \times n$ tripartite graph with minimum degree $n + τ$ must contain $K_{t, t, t}$, conjecturing that $τ= \mathcal{O}(n^{1/2})$ for $t = 2$. We prove that $τ= \mathcal{O}(n^{1 - 1/t})$ which confirms their conjecture and is best possible assuming the widely believed conjecture that the Zarankiewicz number satisfies $z(n; t) = Θ(n^{2 - 1/t})$. Our proof uses a density increment argument. We also construct an infinite family of extremal graphs that are pairwise far apart (requiring the change of $Ω(n^2)$ edges to get between any two).