On any used car I buy with an auto trans I put two cans of trans-x into the transmission and drive it for a while.. then I change out the fluid and filter and put one can in to leave.
Of course you have to pull a little more than the two cans out first...pull out the top if you can... this stuff dissolves varnish which has built up over the years... critical to the gravity working ball check valves in the trans valve body.
I would change out the oil and filter before starting it up...
Have you checked the fluid level in the 3rd member?
Check all fluid levels before getting out on the road...