The Sea King - from memory - has TD and TU with the MoT being an Observer function. He would drive you to a TD position having marked the on-top position on his plot or use a radar fix from the target.
In that respect the Wessex 3 was more advanced. The pilot's HSI incorporated a Nav Display that provided track and distance to a point where you trimmed the IAS hold from 90 its to 70 its then engaged 'Pilot Turn' to auto-turn into the wind (Hdg Bug). When the turn light went out you engaged TD. The definitive point used to set up the circuit pattern was set by the Observer based on either an on-top position or a radar target. The Wx 3 was duplex. The Sea King was simplex and not having done the Phase 5 course yet I can't tell you for sure what the AW139 system is but I will be surprised if it's triplex given that there are only two series actuators in
PR & Y axis and a simplex collective actuator with duplicated inputs.
G.