Those are optical range finders. I'd assume that they make use of triangulation, i.e.: known distance between the two sights + angle to target = range to target.
Kind of useless considering Iowa can make use of radar for target acquisition. However, I'd assume since technical things naturally break, manual range finders are always a good back up.
Also, the reason why they are positioned on turrets 2 and 3 is because it is assumed that the range difference between turrets 1 and 2 would be negligible compared to the range difference between turrets 2 and 3.
Also 2 is better then 1
