Grind rather than cram
If you study French for just an hour per week throughout third year, you will be confident going into the exam and more likely get very good marks! This is a way easier and more successful method than cramming the three-year course in a few weeks before your exam.
That said, keep in mind that it is never too late to start working and that you can still make massive improvements in your last few weeks. I’ve seen some of my classmates jump entire grades in the time between the mocks to the real exam, so don’t be discouraged if you don’t do as well as you were expecting in them. This is actually the entire point of the mock exams - they motivate you to work harder!