If you have received no contact from the agency, you should establish when, where and how they made contact with you.
Could do with more info from you regarding this.

As you have a case with the agency, the only way your other two children will be recognised, is if their PWC opens their own case – on CS2 your liability will be 15% of your net income for 1 child or 25% for 3, less shared care if 52 + nights per year.

Your more than welcome to join us in the forum at http://www.afairercsaforall.co.uk, where we can advise you further.