How safe is it to use 3 ton jack stands to hold up a 3200 lbs. car? I am doing a full rebuild and would prob. Leave it in the air for a couple of months at a time. Or should I use ramps?

I would use neither if it is for months. Jack to the height you need and then slide in soild blocks of wood like a railroad tie, do not use cement blocks. Ramps can still allow a roll back and jack stands are for temporary work and not something I'd get under even for a quick job

It should be fine, If it doesn't look stable, use more blocks.

Hi,
Deere has a commercial machine with three decks and mows about 11' at a time. It weighs over 2 tons and I have never had a problem with it on jackstands, though I often have to lift it in stages. Even as skinny as I am, I can only reach under some if it has the tires on the floor.
Understand that I am using heavier stands than you mention, it does get blocked as well and it is never lifted for more than a few days due to the nature of the machine and the time frame the customer wishes for its return after repair.
Peace,
Clarke