The 3 speed auto is a GM TH180c, and it sounds like you might of just changed trans fluid and all the crud and crap from not servicing the trans is now loose and stuck in the valve body.
The solution is to drop the pan and valve body, clean it out good, replace the gasket if you
break it, and put it back together. sounds scary, but if you are good at mechanics you will be
fine, otherwise find a friend that is.
Good Luck
Wild