Well I unlocked the third ER last night and gameplay is even worse! With 3ER's its now a case of who can barrage your engine rooms first. No one deploys any troops at this stage.
They really need to boost the health of the lower level engine rooms. They are so easy to take down with max art and barrage.
Yes, that's why you need to hurry and get to 4 ER. If you hit all the ER's right when there are three, you can finish is 5-7 seconds. If you are off a little bit, it may take one more artillery that leads to a loss even if you took only 10 seconds.