In practice, I'd expect the three being rotated between the routes as a function of availability, determined by such issues as delays, minor tech problems, maintenance, etc.
Considering:
Singapore - 2320
Heathrow - 0525/1100
Singapore - 0740
Singapore - 2030
Sydney - 0700/0850
Singapore - 1345
we can see that every day from 13:45 to 20:30 there are two planes in Singapore (and the third is in London).
The longest possible stay of any frame in Singapore is 7:40 to 23:20 - from London back to London, 15:40 stay.
Since the Sydney frame and the London frame in Singapore meet in Singapore, they can freely be exchanged or not exchanged, as seems preferrable.
Any maintenance that cannot be done in 15:40 means cancellations - at least till 008 is delivered (when?).