Extracting a maximum from zipped lists
One way to simplify this would be to use nested pairs (('a * 'b) * 'c) instead of a triple ('a * 'b * 'c). That way, you can use fst and snd to get to the parts that you want:

let triplets = List.zip (List.zip A B) C
let best = List.maxBy snd triplets
fst best


Notice that I also changed the names of variables to lowercase, that is the common naming convention in F#.