Firstly, it's about getting the offer, not getting in (whether you get in really depends on your work in year 13, but the offer is subject to how well you do in year 12).
Now, I'd say it depends on what you want to study. Some courses (very, very few) (i.e. economics at Warwick, or another EXTREMELY competative course) will ask for 4 A level grades - and would probably still ask for just a higher grade in the 3 A levels (i.e. A*A*A instead of A*AAA), but otherwise you should be fine.
In regards to flunking one AS and doing well in the rest? A friend 2 years above me got an Oxford offer for French and Spanish, having gotten a U in chemistry AS (and promptly dropping it) - that was because of her other three grades.
(This is because you have the choice to omit any grade you got on your UCAS application, ranging from A right down to U)
I'd say work hard to try and get that C, but honestly, don't stress yourself out too much. If trying to get a B / A will be detremental to your other subjects, then don't do it, and try and spread your time how you see fit.
Like someone said, French isn't particularly related to geography, so I'm sure they'd be ok.
(Also, got an offer from all the Russel group Unis I applied to, having gotten a U at AS chemistry, but an A at AS French - the subject I'm hoping to study)
The point of this long winding text is: you'll be fine