I have a 2002 Century. 200,000 miles.
About 6 months ago I did a pan drop and filter on the transmission. It originally had Dexron 3 in it.
When I did the refill, I used Dexron 6. We are going to replace the car in about 6 months.
It needs a pint of ATF. Is it OK to top it off with Dexron 3, since we are not going to keep the car that much longer?

Will it cause damage when dexron/mercon is mixed with Dexron 6? Likely not. It will likely begin to diminish the Dexron 6 benefits as more and more of it are mixed out however.

Seems like an odd corner to cut right at the very end however.

I don't think it will hurt it any.

Bigger question is where the lost fluid went . . . ?

it will be fine.
ATF is just hydraulic fluid with friction additives.

Well you did a pan drop, which means you only replaced about 2/3 of the fluid, so you still have a lot of 3 in there....so topping off, should be no problem.....