I now have a slow leak in the transmission (pan gasket, etc.) and I'm just going to add transmission fluid periodically as it leaks out.

Not sure what to add though. Manual says Dexron III, but I don't know for sure that's what has been put in it with the two flushes I've had done on it in the past or if it even matters all that much that I use that exact fluid specified.

What's safe to add?

 

I would imagine that the two shops you have previously taken it to used Dextron III as specified. Why not just replace the pan gasket and fill it back up with Dextron and then you'll have no worries?

 

It actually has three external gasket leaks--not just the pan gasket and I have no interest in doing the work on it myself. Don't have the time and don't like working on cars anyway. And I'm definitely not going to be sinking anymore money into this car by paying a mechanic to do it. The car's a piece of crap. I have done every bit of preventive maintenance imaginable on it over the years, all to no avail. About $4,000 in repairs on it so far, with one gd thing after another going wrong with it. And to top it off, it only has 95k on it.

So I'm just going to run it into the ground as a utility vehicle to its last breath and spend as little as possible on it from this point on.

 

Well then I would use a bottle fix with some transmission sealer.