I have a 3-gang metal (ca. 1980, three boxes with removable sides) electrical box that currently has 4 NM cables entering it; at least 3 of them are 12/3, the 4th is either 12/3 or 14/3. All four cables have grounds as well. There are two 3-way switches in the box (a third has been removed and the two cables involved joined with wire nuts).
I'd like to feed a new nearby box with a 12/2 fed from the black and white wires of the two cables that are currently wire-nutted together (and then on to a new light fixture). Would this violate the maximum fill of this box? If so, I can pull that pair of cables from this box, and join them in a new box (this is in a garage, so an awkwardly placed box is not an issue).