Unit 2 helps with Unit 3, in that you explore language in a globalised context. This helps when considering language use in Australian Society, as it provides an understanding of the prestige status of English across time, particular in regards to its role as a lingua franca.
That being said, 2012 will be new Year 12 content, where the focus has changed to language variation for social purposes and language variation and identity. The theory looks much the same, but the order in which it's taught is different and some of the concepts look to be new and/or refined.
In regards to your initial question, you can definitely enter Unit 2 English Language without doing Unit 1, and it's best to do English Language in Year 11 (even 1 of the 2 units) before trying Year 12. While people say you don't "need" Units 1 & 2, the practice of metalanguage you receive in the Year 11 course plays an important role in your success at Year 12.
If you're a very passionate and keen student, entering at Unit 3 is fine, but be prepared to catch up to the rest of the class in regards to metalanguage. If you're just trying to escape mainstream English because your results aren't good, I'd be hesitant to recommend EL at all - you'll work three times harder in EL over mainstream English as the expectations are quite different and for many students quite unfamiliar. If the subject content appeals to you, it's better to enter EL sooner rather than later.
Just my 2c, hope that helps.