I'd probably stick to 3U Math, due to its high scaling (scaling does matter and you will see how it does when you get your ATAR back at the end of this year). Biology is a good subject for Medicine, but I'd probably just do a bridging course for Biology if you really needed it.
Honestly, I think it's much easier to do a bridging course for Biology than it is for 3U Math, because seriously, Biology is a subject where you can just memorise the content and do super well in the Exam. With 3U Maths, you need to consistently and constantly practice it, you'll end up needing quite a bit of support from your peers and teachers and so, it's probalby best to do 3U Maths at school, than at a bridging course.