For a given lattice polytope $P$ in $\mathbb{R}^3$, consider the space $\mathcal{L}_P$ of trivariate polynomials over a finite field $\mathbb{F}_q$, whose Newton polytopes are contained in $P$. We give an upper bound for the maximum number of $\mathbb{F}_q$-zeros of polynomials in $\mathcal{L}_P$ in terms of the Minkowski length of $P$ and $q$, the size of the field. Consequently, this produces lower bounds for the minimum distance of toric codes defined by evaluating elements of $\mathcal{L}_P$ at the points of the algebraic torus $(\mathbb{F}_q^*)^3$. Our approach is based on understanding factorizations of polynomials in $\mathcal{L}_P$ with the largest possible number of non-unit factors. The related combinatorial result that we obtain is a description of Minkowski sums of lattice polytopes contained in $P$ with the largest possible number of non-trivial summands.