If it worked fine before the 12 months of sitting, I would say the sitting
caused the problem.  Seals getting dry and hard and not sealing.  I would
warm it up and drive it around gently at low speed with the shifter in "2"
to get the tranny up to temperature and then see if it starts to engage
3rd.  You may also want to try one of those fluid additives.

Does it work fine other than not shifting into 3rd?

I don't think the engine overheating would cause tranny issues.

Edward

On Wed, May 11, 2016 at 11:32 AM, John Rodgers <jrodgers113@gmail.com>
wrote:

> What would be some causes for the autotranny not to shift into 3rd gear. It
> worked fine before but has been sitting for 12 months!
>
> John
>