sudden tranny slipping 3.8 Supercharged Ultra

What is your question?

Car sat for several months during Seattle cold wet winter months. Upon startup the tranny slipped at take-off. Fluid was a little low so I added Dextron3 to fill line everything seemed normal drove fine. Sat again for two weeks without battery. Upon next startup the tranny slips quite a bit at takeoff and seems to be mushy throughout all gears. Reverse is firm and solid. Thinking time for fluid and filter change . . . if what else ?