Will octahedral splitting occur in tribromidotricyanidocobaltate(III)?
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.