what is the legal definition of child pertaining to child support

In Florida, your child support obligation runs until a minor child turns age 18, or graduates from high school if they are still in school and turn 18 prior to the end of their senior year.   It sounds like your order is being misconstrued, you should not have to keep paying on all three children until the youngest turns 18, rather each child support obligation should end as that particular child reaches 18.  You should file a petition for modification of final judgment, and in that petition, allege that two of the three children are over the age of 18 and have graduated high school, and as such your support obligation should be modified. You will continue paying on the youngest child, as well as any arrears that may be owing on the other children. It is possile that you have over paid on the two older children and may be entitled to a credit against your ongoing obligation for the youngest child.