How much your ranking of 3rd will affect your mark will depend on how well the cohort does and how well you do in external exams.
You should do a subject if you enjoy it (<- most important) and if you are good at it, and you shouldn't drop just because there are people who you think will do better than you.
If it helps, I'm in a very similar position to you with French Continuers. I was maybe 3rd in Year 10, but this year, everyone else in my class except for one other person has either gone on exchange or is a native speaker. Three people have already dropped the subject due to the increase in difficulty from Year 10 and because of how good everyone else is. However, I haven't, and my teacher told me last year that she's had students who have never gone on exchange and are not native speakers top the subject before, and therefore it is definitely possible for you to still do really well in it. I totally understand that with languages, there's a pretty big jump from Year 10 in comparison to other subjects, that it's extremely daunting when there are other people in the class who are far better than you. But if you enjoy the subject, definitely don't give up yet. I myself have put in a lot of effort in the past two weeks and have seen myself improve quite significantly - maybe not yet on par with everyone else, but I've definitely improved, and so will you.
