Can you book mixed cabin itinerary with Qantas (2 stop)?

On Qantas, how do I book different cabin classes for three international flights when booking online? I am wanting to book business for the longest leg, economy for a very short 2hr flight and then business for the third flight.
I want to use points+pay for all flights.
I cannot see how to do this using the multi-city booking so do I just book 3 x individual flights for each class or is there another method?
Which leads to another, is there any problems or implications booking 3 x individual tickets rather than 3 flights on the one itinerary?

Do you mind sharing your route? Is this a 2 stop itinerary (i.e. transiting)? Usually Qantas allow mix cabin redemptions with Qantas points (even when you don’t want this; no way to filter this off). Points + pay is not ideal because it gives you very mediocre value for your Qantas points.