Cuisers won't do any damage in that case. They just will will soak up the damage so your battleship lasts longer so you would still probably win. You suffer much less damage by moving your fleet within range of cruisers and getting battle over with sooner.
Maybe the scenario you gave you would not win. You would hit each of his battleships with 1/3 power and his 3 battleships would hit you with 3/10 ths power so at first you are slightly stonger. But then your cruisers have less hitpoints so they would at a later point disappear leaving only battleships so now it's 3 agaisnt you 1/3. You probably lose. Annialation of both navies.
But you haven't mentioned all destroyers and subs in the mix to take hitpoints so that's all in the mix. Personally if I have more then 10 battleships plus cruisers in the mix I would split up to get all possible fire power on the target. Opponent can only fire on one of your fleets in return. Take him out as quickly as possible to suffer less damage.
