Why Use 3.5C Cables?

You would use 3.5 cables to supply a panel load that is predominately a full three phase, and balanced. A good example would be a motor control centre that will have a low single phase load. I would consider using a 4 core to light and power distribution boards instead of 3.5 because there is more of a possiblity that there would be a substatial single phase load which can become unbalanced easier.