2002 Mercury Sable P0743 P0750 P0760 Transmission Codes - Erratic Shifting and Stalling at Stops

We recently purchased our grandmother's 2002 Mercury Sable with 60,000 miles. Over the past month, the vehicle has exhibited minor erratic shifting behavior. Specifically, when pulling away from a stop sign, the transmission disengages and the car fails to move. The shifter can be manually cycled through all gears, but no response occurs during acceleration. Transmission fluid appears normal in appearance and smell. We took the car to a parts store, where it returned P0743, P0750, and P0760 diagnostic codes. We are unsure if or when the transmission has been serviced. Any suggestions on how to diagnose or resolve these issues would be greatly appreciated.