You will need to do the change manually to make it into a 126 pole unit.
Hi Bot team
John is correct, the designer is not allowing the selection for 3 panel.
when you select the unit with 480v and 3 panel you get an error at the top of the windows that said
(Power is not supported for this main unit. Select a different load, voltage, or main unit)
Please investigate and fix.
We do have a Sku for the 300kva unit with 3 panel (0M-PMM300GF126 300 KVA 480V INPUT 208V OUTPUT 126 POLE)