I need a way to create either a validator or a constraint at model level to evaluate two or more fields to be able to be null/blank only if all of them are null/blank at the same time.

For example, in the next model:

from django.db import models

class Example(models.Model):
    A = models.CharField(max_length=16,blank=True)
    B = models.DateField(null=True,blank=True)
    C = models.FileField(uploadto='/',null=True)

if I try to create a new Example with either B or C values empty it should raise a ValidationError; but if both of them are empty it should be OK.