What type of flight for this, and optimum time to book?

[chorizostew]

DS will finish his final uni exams at the end of May 2025 and wants to spend June/July/Aug travelling before starting work in (probably) September.

He wants to visit Sri Lanka (where he has a friend), then either Vietnam or Thailand, and possibly Australia (depending on time/budget).

Is there a particular type of multi-stop airline ticket for this type of trip, or would it have to be multiple single flights?

Also, if there is an optimum time to book, let me know (otherwise I will assume the sooner the better).

Just do one way fares for each flight.
The time to book is as soon as he has worked out how long he plans to stay in each destination and at least has an idea of accomodation for each stop.

And make sure that travel insurance covers an extended trip length.