Since you can drop a subject at any point during year 12, you may wish to consider proceeding with your current subject combination as is. As you complete your first assessment task/exam for the subjects that you are thinking of dropping and become aware of your performance, you can compare your performance in each. This will allow you to make a more informed decision regarding the subject that you should drop, and that would be the one in which you end up performing worse.
Since there is still time before you start your Mathematics Extension 2 classes, perhaps allocating some time towards familiarising yourself with some of the content (focus on the first topic that you will be taught, if known) would also be good, allowing you to get a head start and possibly leading to a better attempt at your first task, which would help you perform at a higher standard.
I hope this helps!