I have a 1992 Geo tracker convert w/3 speed Auto tyranny and it slips going into third. I have replaced the Vacuum module because it had a leak back into the intake manifold and blew white smoke out the tail pipe at times. I also blew out vacuum line to ensure fluid residue was removed.  First and second gears seem to work. Also replaced the tyranny filter thinking it might be clogged.  Ecm was rebuilt and instaled and no change as the rebuilder said the ecm does not operate any part of the tranny. Shall I start looking for a used tranny

What might I try next???

The valve body gets stuff stuck in it sometimes, pull it apart, or have it done
and the trans should give you lots more service.

Wild

  the 3 speed auto in my 91 kick was doing the same and more.I had it flushed and a cleaner run through it to free up sticky valves,and it works perfect.It ran me about $100.Worth every penny.

I heard that the "Seafoam" tranny flush works good. I had a 3-speed auto that was on its last legs and got a few thousand more miles from it by putting 6 oz of brake fluid in the tranny. I wouldn't recommend this for a tranny you plan on keeping though because I don't know if this would damage the unit permanently.

Junkman

Brake fluid is not an oil, and doesn't mix in trans fluid.
Do the good old fashoned  flush or tear the valve body down.