The single-phase loads in a facility, such as lighting, are divided among the phases of the in coming 3-phase power supply. The load is rarely divided precisely. The differences in phase current produce differences in phase voltage, especially if the conductors are heavily loaded
The appropriate solution may be to redistribute the single-phase loads among the phases, to increase the capacity of heavily loaded conductors, or to install separate transformers for 3-phase loads.
It is usually not a good idea to balance phase voltage by using the transformer taps. If the phase imbalance is so bad that transformer taps would make a difference, the problem is bad enough that you should correct the underlying problem.