How are load currents balanced in a distribution system?
In this case, will the system automatically equalize Ia, Ib and Ic
  again for the sake of a balanced system?


No it won't automatically balance and, as a result there will be some current passing down the neutral wire to the 3 phase power source. Previously, with all three loads balanced, neutral current would have been zero.


  How are load currents balanced in a distribution system?


Statistically, with many households and factories, the currents are reasonably balanced in full load scenarios but nothing is perfect and current imbalances do take place.