Q: automatic transmission works fine in reverse, 1st, and 2nd, but revs the engine in 3rd 4th 5th only goes 30mph

You could have a defective vehicle speed sensor, transmission shift solenoids, transmission control module, valve body in transmission, throttle position sensor and/or the interconnecting wiring, fuses, and terminals (involving the electrical components) could have a defect. Another possibility is an internal hydraulic leak in the circuit that takes over once 3rd gear comes into play. The Factory Service Manual sets out a basic decision tree to narrow the diagnostic. You can simply apply that tree or to make quicker work of this, YourMechanic can dispatch a certified mechanic to perform the diagnostic. Some transmission faults, particularly electrical in nature, but also including almost anything to do with the internal valve body, can be repaired without removing the transmission. If, though, an internal piston has failed or a hydraulic circuit is leaking, usually it most efficient to just simply rebuild the whole transmission as it has to be removed from the car anyway to perform the latter category of repairs (i.e., piston, seal failure, etc.). There’s always a chance of a relatively simple cause, so I do encourage you to request a transmission shift diagnostic to see if this can be resolved on the car. If you have further questions or concerns, do not hesitate to re-contact YourMechanic as we are always here to help you.