In a previous HSC, they have once asked for 3 poems which my teacher thought was absurd, due to the fact that we are expected to have 2 readings for each poem. So 6 readings in 40 mins?? zzz
My teacher stated that this rare case does happen, then you should go through 2 poems in detail and probably one paragraph briefly on another poem. Make sure you answer the question as well.
AND yes, they can specifically ask you to do a certain poem. MOST likely, they will say "..make detailed reference to poem A or B and a poem of your own choosing "
Hope that helps