Dexron 3 will do just fine.
On my 1993 Topaz, the transmission fluid dipstick is on the right side of the engine, near the front, just to the left of the battery. It looks a lot like the oil dipstick, but, when you pull it out, the writing on the dipstick clearly mentions that the fluid level should be checked when the engine is hot and idling, and that a certain type of Mercury automatic transmission fluid should be used to refill.
According to the 2000 Mercury Sable Owner Guide : ( Motorcraft MERCON - V , automatic transmission fluid in both types of automatic transaxle / transmission )
The Mercury Sable transmission has a fluid capacity of 12 quarts. You can use any SAE certified automatic transmission fluid.
Motorcraft MERCON automatic transmission fluid for the automatic transaxle ( according to the owners manual )
The automatic transmission takes Motorcraft MERCON - " V " - automatic transmission fluid The transfer case takes Motorcraft MERCON - automatic transmission fluid
According to the 1999 Mercury Sable Owner Guide : ( Motorcraft MERCON-V automatic transmission fluid ) ( to confirm the type used , it should be engraved in the automatic transmission fluid dipstick )
Motorcraft MERCON V - automatic transmission fluid , according to the owners manual
According to the 1997 Mercury Tracer Owner Guide : For the automatic and manual transaxle ( transmission ) ( Motorcraft MERCON Multi Purpose automatic transmission fluid )
it uses Mercon V at transmission fluid. both automatic and manual transmission
The 2004 Mercury Mountaineer does not have an automatic transmission fluid dipstick . I know that the 2002 and newer Ford Explorer four door versions do not have a dipstick for the automatic transmission fluid level .
Motorcraft MERCON - V , automatic transmission fluid in the automatic transmissions
Motorcraft MERCON automatic transmission fluid for the automatic and 5 speed manual transaxles according to the owners manual