Да, вы можете переопределить в методе save ().
class Person(models.Model):
name = models.CharField()
first_phone = models.CharField() # <-- user fills only single input: 12345
second_phone = models.CharField() # <-- auto set using field: first_phone
third_phone = models.CharField()
def save(self, *args, **kwargs):
phone = self.first_phone
self.second_phone = phone
self.third_phone = phone
super(Person, self).save(*args, **kwargs)