Legal Question in Credit and Debt Law in California

I'm not sure what you mean by a "third party," but a collection agency as agent of the creditor can file suit on behalf of the creditor if the creditor authorizes it. In addition, it sounds more like you are dealing with junk debt buyers. In that case they have bought the right to collect the debt from Discover, and that makes them the owner of the debt, not a third party. They have four years from the last transaction on the account (charge or payment) to sue to collect the debt.