Do Cambridge look at your best 3 A levels if you've done 4 A2s?

I could give my guesstimates which is just me guessing on what I think randomly

or you could ask a Cambridge admissions tutor here,

http://www.thestudentroom.co.uk/showthread.php?t=4327946

I'm reapplying this year for medicine after getting my A2s, and I have A*A*AB, with the B being in geography. Would they not look at the B in geography, because if they do it's not going to do any favours for me. My UMS averages are high for the other 3 subjects, and I just missed out on an A* in maths by a few ums. How realistic is my application?

The "best 3" thing is really only used when calculating UMS averages for "auto-pooling". However auto-pooling doesn't apply to Medicine so Best 3 is not relevant to your application. Also for medicine it would be Best 3 SUMS (i.e. sciences) so geography wouldn't apply anyway.

Also, you have A*A*A(B) which meets the typical requirements for Medicine so you will be reasonably competitive from that perspective. However note that most post A-level applicants exceed the typical requirements. But if the other aspects of your application are strong (BMAT, etc, etc) then you can still be successful.