lived in tenn and divorced from ex in 1998, mda stated i pay her 42% of net income for three kids. In 2003, when our kids were 21,18, and 16, ex got a income assignment for $1,000 a month for child support. the kids are now 22,21, and 18 and out of high school.

Where do i have to file, and what do i need to file in order to get my support order terminated?

I'd call the office your support order originated from. They will be able to tell you exactly what you need to do.

Review the order too - it may just say "until the age of" and you wouldn't have to do anything.

if kids are attending college or post secondary schooling full-time, the support goes on; if not, you should file a motion to terminate the support order as to all 3 kids in the court where the support order was issued; I suggest hiring an attorney local to the court to do this