exception CombineError

let rec split3 l =
  match l with
    [] -> ([],[],[])
  | (a,b,c)::t -> begin
                  let (al,bl,cl) = split3 t
                  in (a::al,b::bl,c::cl)
                  end

let rec combine3 al bl cl =
  match (al,bl,cl) with
    ([],[],[]) -> []
  | (ah::at,bh::bt,ch::ct) -> (ah,bh,ch)::(combine3 at bt ct)
  | _ -> raise CombineError