Wow you are in quite a pickle, but don't stress too much. I am doing spec and methods so will give my 2 cents on those, and I do know one kid doing general 3/4, methods 1/2 and is planning to do spec 1/2 in semester 2.
So the easiest one I will talk about first is doing spec. Do it. Trust me, it is overestimated and quite fun. To get a real sense of the difference between spec and methods, methods will ask you to use a special technique to solve some wordy problem. Spec on the other hand, will simply give you a straight forward (make no mistake, these are typically very difficult, like 10 questions every 2 hours on the exam hard) but you can solve it using ANY METHOD at all. Best example I can thing of, is a geometry one, so something like 2 circles intersecting a square with a square that touches both circles inside the square, find the area of the square. Kinda difficult to imagine however the main point is it can be solved many ways, using pythag to find the length missing from both sides, make the circles and squares into graphs etc. It is a free for all any way you can think of is not only valid but encouraged. It also covers a wide range of topics you have never seen before and is very useful for engineering and physics. It also scales a lot. So in short, if u enjoy math then do it.
Methods in my opinion, is a bit harder. It requires you to understand formulas, methods of working out where to apply them etc and also some theory. Honestly, just doing a lot of questions and keeping a note book of methods of different types of questions will get someone of your skill through the 1/2 fairly well. Just make sure not to fall behind, it can be quite fast for some kids. One thing with methods, is that spec makes it around 1 billion times easier. For example, over 1 term of year 12 spec is calculus and trig, one large part of methods is calculus and trig. If you can get good at the spec stuff, then suddenly half of methods becomes revision, and easier revision at that. However, methods is probably one of the most interconnected subjects in all of VCE. You can't go half a term without covering something needed the next year. So missing out on unit one could prove a big challenge, but once again, it depends on you.
So yeah, methods and spec r a great combo. But now we have to look at general... so yeah, the general stuff is (sorry if this offends anyone) easy. In terms of math, the concepts r not abstract and sometimes repetitive. Interestingly, spec does cover parts of the general course haha, just some of the trig, finance and matrices. I know one kid doing accelerated methods and 1/2 general, he is super lazy, like big time but he gets minimum 90-100's because when you get the math, and know how to watch out for mistakes that separate the good students from the great ones, I have heard it is fairly easy in terms of content and work load.
One more thing to consider is the teacher and class size. If you have a small spec class and a great teacher, he/she may be willing to lend you a hand with methods and general stuff. So make sure u get an idea of who is teaching/doing the subject as well.
But bottom line is you do not need all 3 subjects. Sorry, but general is great for finance and certain fields but as you said, not needed for you desired course. methods and spec really should be taken in conjunction and cover a huge field of math. So whilst I believe you are capable of doing all 3, whether or not you need to is another question. However, as you said you may not even be able to transfer out of methods, so here is my final, summed up conclusion having talked about both sides.
Do all three. Seriously, if you enjoy math (seems like you do), want to challenge yourself, get exposure for a 3/4 (worth it), have a good class or teachers and can't even drop general for methods anyway (wouldn't recommend halfway through the year) I think it may be worth your while doing all three. Spec 1/2 is so stupidly unrelated to the 3/4 it is hilarious but the methods stuff is so connected it could be a problem, but managing all three of them is manageable with some effort and learning some effective study, also it is only year 11 for 2 of them so all you need is an S. Of course, if you find yourself hating general a lot then transfer out because hating a subject sucks but if it is just boring/fine maybe try sticking it out, some stuff will get hard and trip you up. Make sure you know whether you want to do another 3/4 in year 12 or just take some free periods. But yeah, just stay positive, have fun and if your year 11 is super hard, just think about all the suckers who walk into year 12 not knowing how to work unlike u

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Have a good one.