I don't like to use other than what is reccomended. However, Mercon 5 is a synthetic Mercon 3 and most techs will say they are interchangeable. I find so much conflicting information that I would ask a tranmission expert. If you did not have a transmission shop work on it, I would ask a local transmission shop what there opinion is. Automatic tranmissions are very hard to work on and take a great deal of expertise to work on. That is why you find places like AAMCO that only work on tranmissions and can ask an expert their opinion as they will actually know from experience or have a factory manual to refer to. Our service manual does not. So, you could go to a local parts tore and look at a bottle of Mercon 5 and it should say that it is interchangeable with Mercon 3 and/or meets its standards. Or go to a local tranmission repair facility and ask them if they thinik it is okay.
I believe it is okay but I am not a tranmission tech. I would double check it with an expert in tranmsissions or verify it on a bottle.
If you are told it is not interchangeable or do not find that Dexron 5 bottles do not say that they aare compatible or meet Dexron 3 specs, you should change the fluid as soon as you can to be safe.