3 oz should have no affect, transmission fluid expands a good bit when it comes up to full hot temp and the systems are designed to leave room for that expansion you should be within the design specs if only 3 ozs over full. Unless this is a teeny tiny trans on a little bitty bug car that only holds a couple of quarts of fluid total or sumpin. Probably 90% of the vehicles that go thru the quick oil change places are over juiced, those high school kids that work there could not care less lol

dnt1,

Thanks for the response. The transmission is in at 2004 Forester, so the transmission isn't small.

I had checked the transmission hot as part of my oil change. It read a little below the full for hot (but how hot is hot?), anyway I decided to add just a little, in fact the 1Q bottle of Dexron III I had on hand was almost empty and I used only a part of that, trying to be careful not to overfill. I put a oz or so in and check, still a little low, put in another oz or two and quit. Later I check it cold and it read overfilled for full, way over the cold range. I don't see any fluid on the floor of my garage, nor do I smell anything unusual when driving the car. I'll check again but think I must have had a some problem with proper cleaning of the dipstick before making the reading.

Ive been to another forum and there are different problems about overfilling the transmission.. It depends from tranny to tranny.. from a tranny chop FAQ: they say, it wont hurt the transmission though it might cause some parts to move and become aerated.. And of course leaks that are not really so ordinary would be visible..

Here is a link that might be useful: Automatic Transmission Solenoid