Isolating polarities on a series wired battery bank
If you want to connect three 12-volt batteries in series to create a nominal 36 volt battery bank, you simply connect the positive terminal of the bottom (most negative) battery to the negative terminal of the middle battery, and the positive of the middle battery to the negative of the top battery.  The negative terminal of the bottom battery will be the negative terminal of the full bank, and the positive terminal of the top battery will be the positive terminal of the bank.  There is no need for any additional components between or around the batteries.