The ST-3's that I have experience with all can not tolerate ANY load imbalance. Nothing to be done except to run either 230 or 120V. If you need both, step up or down via transformer for the smaller load, or step down near point of use if going some distance. I recommend Antekinc.com for high efficiency toroidal transformers.
https://www.antekinc.com/an-154115-1500va-115v-transformer/I have also used smaller transformers in parallel to increase capacity with a suitable fuse for each transformer. The Antek transformers will handle huge surge loads. A warning- because of the high efficiency wound silicon steel cores, they do have high inrush current so soft start may be required on something like an inverter. The ST-3 has no trouble with them.
I will note that I have seen significant variation in ST-3 stators, in particular, along with great differences in harmonic winding and clearance between rotor and stator. These affect waveform distortions, regulation, etc. A pity no one has yet identified a single source in China that is reliable and has consistent higher quality.