  For some theories where the degrees of freedom are tensors of rank $3$ or
higher, there exist solvable large $N$ limits dominated by the melonic
diagrams. Simple examples are provided by models containing one rank-$3$ tensor
in the tri-fundamental representation of the $O(N)^3$ symmetry group. When the
quartic interaction is assumed to have a special tetrahedral index structure,
the coupling constant $g$ must be scaled as $N^{-3/2}$ in the melonic large $N$
limit. In this paper we consider the combinatorics of a large $N$ theory of one
fully symmetric and traceless rank-$3$ tensor with the tetrahedral quartic
interaction; this model has a single $O(N)$ symmetry group. We explicitly
calculate all the vacuum diagrams up to order $g^8$, as well as some diagrams
of higher order, and find that in the large $N$ limit where $g^2 N^3$ is held
fixed only the melonic diagrams survive. While some non-melonic diagrams are
enhanced in the $O(N)$ symmetric theory compared to the $O(N)^3$ one, we have
not found any diagrams where this enhancement is strong enough to make them
comparable with the melonic ones. Motivated by these results, we conjecture
that the model of a real rank-$3$ symmetric traceless tensor possesses a smooth
large $N$ limit where $g^2 N^3$ is held fixed and all the contributing diagrams
are melonic. A feature of the symmetric traceless tensor models is that some
vacuum diagrams containing odd numbers of vertices are suppressed only by
$N^{-1/2}$ relative to the melonic graphs.
