I need help with ocaml

Can u help me gus. I have task: “write a function that takes three arrays in a tuple (array1, array2, array3) and returns the longest common
subsequence of the arrays”. I dont have idea how to do .

let longest n = fun a1 a2 a3  ->
    let a = Array.make  (Array.length a1 + Array.length a2 + Array.length a3) a1.(0)) in


What effect should n have on the result? What should happen if the arrays are of different lengths? If there is no longest subsequence? If there is more than one subsequence of equal length? Try to specify the requirements exactly and keep them all in mind while implementing.