A 100 kVA transformer can satisfactorily take 33.3 kVA single phase load on each of its three phases if it is fully balanced. On the other hand it cannot take even 50 kVA load if it is connected on one phase only. This embosses the balancing of the loads equally between the phases. Therefore never add or remove load for effective utilization of transformers and machines.
Remember, unbalancing leads to burning out of bulbs, house hold appliances and machines too.