Welcome to the forums Lyly78!
First of all, I totally get what it's like to not meet a goal in a tough subject like 3U. I only just scraped a pass in my 3U half yearly exam and ended up coming 2nd last in the cohort. By the end of the HSC I was in the 90% range and in 1st place - I treated the poorer performance as a motivator to work hard and do better!
So my advice would probably be to keep going, provided you are still enjoying 3U and feel okay about the content! If it is just silly mistakes in exams,
you need more practice under exam conditions. It's all well and good to do well in textbook exercises, actually putting on a timer and doing a practice test under pressure is tougher. Practicing under those tougher conditions should be a priority for you, if it isn't already
Good luck!! You've got this