Truck sat for 3 years now transmission slips in forward but not in reverse

Sitting up that long would suggest that some of the parts in the transmission are probably sticking. Is the vehicle ran just fine before it was parked that's all that is, but if unknown condition when parked, could be at the least stuck parts. Would not suggest using any transmission slip product. Check the fluid, and the filter, probably would flush the transmission. Go from there. A '95 F 250 ,assuming with it automatic transmission is about ready for an overhaul