Hi there,
If your university only uses your third year grades to finalise your grade, then yes, as you calculated, you need to achieve at least 61 in the 3 remaining modules to achieve a 2:1 or at least 82 in the 3 remaining modules to achieve a 1st.
Now I know it's quite stressful, but I also want to remind you that the employers won't just be looking at your grades when examining your CV. You could still be an amazing applicant, if you achieved a 2:2 in your degree yet have so many work experiences and skills that make you distinct from all the other applicants, some who may have achieved a 1st yet have no experience at all. And remember network and be yourself. If you can show that your a likeable individual to your future employer, you've made it!
Good luck.
Regards,
Narusha
Coventry University Student Ambassador