It’s great to hear that you’re doing some Scholarship subjects in Year 12 - it’s a really good idea to get started early with these exams and not just leave them till Year 13 if you really want to do well. I did a few Scholarships and self studied most of them so should be able to give you a couple pointers.
Firstly, definitely do Calculus and Economics if you’re studying them at L3. Statistics also makes a lot of sense to do and it’s one of the easier Scholarships to learn and get.
With Chemistry and Physics, these can be good ones to do but be wary that you will need a really solid grasp of Level 3 concepts. This means that you’re going to have to learn all the Level 3 content for both of these. With that in mind, I’d recommend doing one of these at most (probably the one you’re most interested in, but keep in mind that Physics is likely to be a little easier if your Maths is quite strong).
This leaves the essay based subjects. PE is a really good one to do because there’s no exam. You just pick a topic in PE/Health (really wide scope) and then write a report about it (examples are available on the NZQA website to give you a pointer). This means you can do this early in the year and get it out of the way.
Geography and Art History are also reasonably good ones to try. I haven’t done either, but from what I gather, Art History requires a lot more knowledge and is generally harder, whereas Geography has a slightly lower standard and since they give you resources in the exam there isn’t as much to learn (it’s more about the skill of answering questions).
A couple of other ones to consider are English and Earth and Space Science. English Scholarship tends to have a reasonably high standard, but by doing the scholarship your writing skill and style will be pushed, as will the way you think about texts and the texts you engage with, so it can be a good one to improve your writing and skill for Level 2/3 English. Earth and Space Science is another Scholarship where you write three quite brief essays on topics about the ocean, the atmosphere and outer space. This is a really good one to self-study because there’s not a lot of content and if you are interested in science it can be really interesting to learn. The exam itself isn’t very hard and they give you some specific resources in the exam with which to develop your essay responses.
Hopefully that gives you a bit to think about!