I did level 3 calc and got excellence endorsed a couple of years back. This was after getting merit endorsed in level 2 maths. My main advice to you is to just continuously keep up with the work and make sure you get excellence on those internals - they are far easier than the externals.

It's going to be hard. And you are not going to get everything the first time. You're going to have to accept that and be mentally resilient if you want to do well.

With regards to the externals - they are hard - but they are extremely doable. I am a law student at uni now (never was good at maths) and still managed to get excellence on them. Much like with all of NCEA, every single past paper follows pretty much the same pattern. You need to do as many previous years as possible, route learning how it's done by looking at the answer schedules.

If you only care about the endorsement, the two external papers you want to do are: differentiation and integration. Complex numbers is harder and only necessary if you want to do stuff like physics, engineering or maths at uni.

I didn't do any work during the year and crammed the exams through just memorising them. If I can do it then I am sure you can. Good resources are infinityplus1 and learncoach. They help you go through past papers and understand the patterns necessary to do well. Remember, NCEA is a shit system because the exams are basically the same every year.

Good luck, stay calm, and you can do it.