You can apply for services with the OAG for free to ask for a reduction based on changed circumstances. You don't say if you are getting child support from the other parent of the 2 children you have custody of -- if not, you should! If you owe back support on child no. 3 you will still have to pay it even if child support is reduced. You can hire a private attorney to do it faster but you will have to pay money up front. Good luck.
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Your support should drop, however this is only if the two children you say you have custody of now are yours biologically or through legal custody orders. You can go the OAG route or hire a private attorney (which is quicker.)
I practice in Tarrant, Dallas, Denton, Parker and Wise counties. I am licensed only in Texas. Answers are based on current Texas law. This is for educational purposes only. This answer is given based on the facts provided above. Contact a local attorney to further discuss these issues. No attorney client relationship is established. These questions and answers are fully searchable and viewable on the internet.