Guideline child support for two children when you are supporting one other child at home is 22.5% of your income. That is probably what is in your orders. If your orders were drafted before you were supporting that third child, they may have been set at 25% (the amount for two children when NOT supporting any other kids). If that is the case, and your income has NOT gone up since those orders, you may with to contact the Office of the Attorney General of Texas and ask for an adjustment. I'd suggest you first request a consultation with a private attorney, though. It's worth the consultation fee to have someone go over the numbers with you to make sure you'd be better off asking for an adjustment.
Note that the payment of the arrears might make the total (current support plus arrears) more than 22.5% of your income. It's not supposed to go above 50% of your income even with arrears, though.
The above is legal information, not legal advice. If you have not paid me and signed a contract, we do not have an attorney-client relationship and I am not your lawyer.