If your incoming supply gets as low as 100V...
and you want to step it up to 230...
Then you're asking a great deal from the existing equipment. It will have to carry twice the normal amps all the way to your step-up transformer.
Because 60,000W at 230V is 261 amps. At 100V it will be 600 amps. How easily can the existing equipment handle that? You must also factor in I*squared*R losses, as heat generation.
I imagine you expect your 3-phase motors will use less power once they get the normal 230 volt supply again. As an experiment you might see if it helps things along by boosting just one loop up to 230V.
For small changes up or down, there are transformers that have multiple taps so you can obtain a higher or lower voltage.
You probably can get by with an autoformer. It has a coil which is included in both the primary and secondary sides. Expense is reduced. However you won't have isolation of one side from the other.
For 3-phase you'll need 3 of them.
A 60,000W unit is industrial strength. Big as a desk.
You want your equipment to detect when the power company resumes 230 V. Because if your incoming jumps suddenly from 100 to 230, you'll get a lot of burned-up units. To respond automatically when your incoming changes, you'll need massive relays which switch multiple transformers in and out of the loop. This will be difficult to accomplish, and expensive. I don't know whether there is equipment that can do it smoothly and without power interruptions.
Your electric company must be having severe problems. Have you considered generating a portion of your electric power on site?