Hey, i have a 1988 gl wagon and recently it started popping out of 3rd gear when i am accelerating

Any ideas? 

I plan to change the tranny fluid for now but not sure what the issue is

Could be weak engine and transmission mounts causing the drivetrain to shift under acceleration. Possibly also shifter linkage/bushings. Either that or the bearings in the trans are clapped. 

GD

Or a broken retainer spring inside the gearbox. There are three or four 10mm plug bolts on the side of the gearbox close together. These have the retainer springs and balls behind them. 

Use a hooked piece of wire to pull the springs out. Compare them all together - they should be the same size. 

Check what GD mentioned first as these are easier to do. 

Cheers 

Bennie