I'm not sure how your power is metered, but this is possible (it would be a question for your power company as to how your usage is metered - peak phase, averaged across all 3 legs, or total consumption (each phase metered & summed)).
Regardless of utility costs though, you should speak with an electrician and balance your load across all three phases for the other reason you mentioned - It's kinder to your UPS equipment.
Datacenter grade UPS systems are EXPENSIVE to service/repair, and the maintenance windows almost always require putting the UPS into bypass mode (which means no power protection). Re-balancing your load will reduce heat and other stresses on the UPS system and improve your overall operating efficiency.
Additionally I've found it's easier to plan new build-outs and added capacity if your power load is evenly distributed (design each build-out to balance the new load), or at least arranged according to a design plan that splits it up more than what you seem to have.