Hey,
So it's good that your already thinking about career paths and what you need to get in. Honestly it's too early to stress about ucas especially since you've just started year 12 so ignore your teachers about it for now.
First off, I think you should think carefully about what subjects you'd ultimately want to do whether it's changing one or 3 of your subjects. For example, if you didn't enjoy maths/ do too well on it you might struggle more in a levels but that's up to how you approach it.
So here are some options if you do want to change/ add subjects.
First is to repeat year 12 with subjects you want to do. This may seem like a drastic thing and that your falling behind but in the long run one year isn't making a difference. You'll see this when you enter uni eventually. Also since you haven't sat any exams, it won't disadvantage you against others applying for the course if you stick with some of the same subjects.
Another option is to sit maths privately but maybe after your year 13 exams. Taking a year out or something similar.
Third option is a foundation degree (after year 13). Many unis offer this if you have a different combination of a levels than the one needed for a course.
If your considering law, do more research as it is a saturated and competitive field. Do some work experience to see if it aligns with your interests.
All in all your not a failure and don't stress yourself out about it too much.
Good luck and feel free to ask any more questions.