Will octahedral splitting occur in tribromidotricyanidocobaltate(III)? Octahedral splitting occurs strong field ligands are coordinated with the central atom. What if we have both types of ligands attached to the central atom as in $\ce{[Co(CN)3(Br)3]^{3-}}$, knowing that $\ce{CN^-}$ is a strong field ligand while $\ce{Br^-}$ is a weak field ligand.
In reality it would perhaps boil down to experimental reasoning, but if it's an exam asking you this, I'd like you to remember a rule of thumb as you'd call it:


If the complex has a coordination number of $6$, with $3$ strong field ligands and $3$ weak field ligands, then consider all the ligands to be strong field ligands.
If the complex has a coordination number of $4$, with $2$ strong field ligands and $2$ weak field ligands, then consider all the ligands to be weak field ligands.