Ex claimed my son as a dependent?

yes, you can just file your tax with the two other children, and amend your return and send it in before april 18. After that the irs will ask for the 3rd child record, school etc.. see who will have the legal right to claim the child on the tax for the credit. After all that is solve a check will be mail for the difference. Time wise it would take around 6-12 weeks before you see the check from the irs.