  We consider a multi-scalar field theory with either short-range or long-range
free action and with quartic interactions that are invariant under
$O(N_1)\times O(N_2) \times O(N_3)$ transformations, of which the scalar fields
form a tri-fundamental representation. We study the renormalization group fixed
points at two loops at finite $N$ and in various large-$N$ scaling limits for
small $\epsilon$, the latter being either the deviation from the critical
dimension or from the critical scaling of the free propagator. In particular,
for the homogeneous case $N_i = N$ for $i=1,2,3$, we study the subleading
corrections to previously known fixed points. In the short-range model, for
$\epsilon N^2\gg 1$, we find complex fixed points with non-zero tetrahedral
coupling, that at leading order reproduce the results of arXiv:1707.03866 ; the
main novelty at next-to-leading order is that the critical exponents acquire a
real part, thus allowing a correct identification of some fixed points as IR
stable. In the long-range model, for $\epsilon N \ll 1 $, we find again complex
fixed points with non-zero tetrahedral coupling, that at leading order
reproduce the line of stable fixed points of arXiv:1903.03578; at
next-to-leading order, this is reduced to a discrete set of stable fixed
points. One difference between the short-range and long-range cases is that, in
the former the critical exponents are purely imaginary at leading-order and
gain a real part at next-to-leading order, while for the latter the situation
is reversed.
