What makes you say that? Your child support scales with your income, so you should have more income with a $3/hour raise. If you do not, then your other deductions are higher. It's quite possible to change employers, move to a job with a higher pay level, and come home with less money if they charge more for health insurance, LTD/ STD insurance, etc.
Your child needs to be supported - and if you were the custodial parent your child would benefit from a pay increase as more money would be spent to support your child. Why should that be different when you as a non-custodial parent receives a pay increase?