Usually over 3 segments doesn't work well. I had no problem using multi city to book SYD-BKK-PEK-LAX. I did for some strange reason be able to book SIN-BKK-LHR-ZRH-BOS once but it was almost impossible and i was not able to recreate similar bookings.

Book to ICN and call in to add the final segment and there is no phone charge