Wouldnt only ONE other related text be, insufficient for discussion? Because the syllabus says a variety of texts and if you only mention three- thats not exactly a variety? I asked one of the english teachers from the HSC help line, and she said to definitely do two. However, if you can analyse your one ort extensively, then you can still get a really good mark cos the question only specifies "at least ONE" She also said that you can manage to do more than two- if you can give all texts an equal amount of analysis in the exam, and she said that students under stress will obviously find it difficult to maintain balance within 5texts in the space of 40mins.
Either way, i personally think that if you can give direct links from the text to your prescribed journey focus, give links between texts, bounce from argument to argument and each text flow nicely to prove your thesis- than the number of ORTs doesnt really matter. I dont think they will ask for TWO, but seeing as they will purposely make the HSC harder this year: who knows?! Better prepared than not.