they usually ask for make reference to two or AT LEAST two prescribed texts. therefore, 2 is sufficient (and most people do two - since you also have to do a related).
However, they can technically ask you to write a specific text, but this is very unlikely. Take for example my mod b task for ts eliot, (eng adv tho) they can prescribe any poem (and did so in our trials).
But ever since the new syllabus came out, they've only asked for two and one related. I think its pretty safe to do 2 for trials, but for the sake of the hsc maybe prepare for all 3, because there was 1 year (before syllabus change) that asked for reference to 2 related texts. (I know this is related texts rather than prescribed, but nothing is stopping them from doing the same thing for prescribed too)