transmission not shifting into third, if it does it slips pretty bad. on 1993 Chevrolet G Series Van (G20)

put tranny fluid in it then its not showing on dip stick after driving for a few minutes, cannot locate any leak

First thing to do is fill the trans. to proper level!!! Check the fliud with trans. in park eng. at idle. It has to be either leaking or burning it through modulator valve.
Look on the back of the van see any spots? If so that is transmission fluid LEAKING! If not pull hose off of mod. valve to see if fluid comes out, it should not! Also check for fluid in coolant. If none of this makes any sense to you seek a mechanic to look at it.