def validate_password(password):
    if len(password) < 6:
        raise ValidationError(_("Password must be at least 6 characters long."))

    has_letter = False
    has_digit = False
    has_underscore = False

    for char in password:
        if char.isalpha():
            has_letter = True
        elif char.isdigit():
            has_digit = True
        elif char == '_':
            has_underscore = True

    if not has_letter:
        raise ValidationError(_("Password must contain at least one letter."))
    
    if not has_digit:
        raise ValidationError(_("Password must contain at least one digit."))
    
    if not has_underscore:
        raise ValidationError(_("Password must contain at least one underscore."))