There are multiple students in my grade doing this type of subject selection. They are doing quite well. There have been past HSC students in my school who have done this combination and have done exceptionally well too. The only difference they do thirteen units (we have to do religion at my school) and non of them do all three sciences. Personally, I think if you can manage your time well, you'll be fine. Don't leave everything to the last minunte is my only advice. I think you can handle the workload. If you are in the situation of wanting to drop during the year, you have an advantage. You can drop a two unit subject and still have twelve units, or drop a one unit subject and have thirteen units. You have placed yourself in a good position too.
I do Maths Extension, some topics are challenging but if you work hard you'll be fine. DO NOT EXPECT THE SAME RESULTS YOU WILL GET IN YEAR 10. Majority of my class received a rude shock with our first two assessments. HOWEVER, THIS DOESN'T MEAN YOU WILL STRUGGLE. If you work hard, you will be surprised with your marks, in a very pleasing way. Trust me with this. Past paper, past paper, past paper. Expose yourself to some challenging questions. This will prepare you for the harder questions at the end of the paper. This is my tip for studying for exams, as well as many other forum users.
Chemistry - in my opinion, I'm struggling to enjoy this subject. I personally see it as a do it yourself subject because I don't find the teacher's teaching too useful. Generally I look up tutorials to develop an understanding of the terms and concepts. There are some good resources on this website too with syllabus dot point notes, that really go in depth with the dot point explanations. This has made Chemistry a little less tedious. However, since you are doing three sciences, I don't think you should stress. Different people have different experiences. There are many, many, people who have had a good experience on this forum, I'm one of the not as many people who haven't enjoyed their experience so far. You have a very big passion for science and seem to be one of those people who can understand all the concepts. Good on you for this!! : ) Also your passion for science will motivate you to work harder, so you will do better in the assessments.
By the way - you generally have four to five weeks of first term to change subjects if you decide to. If it becomes too much of a workload for you during the first two to three weeks you can always change subjects, or drop a subject.
All the best. : )
