Yep thats true, im in yr 12 there, the only 3/4's you can do are accounting, psych, biology, accounting, music performance (i think) and maybe one or two others.
The reason is that they think that for the harder subjects, like methods 3/4, unit 1+2 does help a lot. For instance calculus is introduced in 1+2, and without that introduction, 3/4 calculus can be difficult to grasp. Same with probability etc.
Don't immediately say 'thats bullshit', try looking at it logically, and the above makes it an understandable decision. And no there is no way they would let you, Although recently they have been taking feedback on general running of the school, and this topic they say came up a lot, and they are reviewing their policies.