I strongly suggest that you use only Mercon V fluid.
Not mercon III.
Drain the torque converter, and install a new filter and a transmission cooler when you are servicing it.
Read the tech article from the front page, as to how to properly drain your transmission fluid. Just dropping the pan won't even get close to all the fluid. If you follow the tech article, you will get all the fluid except for a quart or so out of the transmission.