A distribution on triples with maximum entropy marginal

We construct an $S_3$-symmetric probability distribution on $\{(a,b,c) \in \mathbb{Z}_{\geq 0}^3 \: : \: a+b+c =n \}$ such that its marginal achieves the maximum entropy among all probability distributions on $\{0,1,\ldots,n\}$ with mean $n/3$. Existence of such a distribution verifies a conjecture of Kleinberg, Sawin and Speyer, which is motivated by the study of sum-free sets.