I agree with others.. your figures are out a bit. I have 2 kids and get the max rate of everything and i'm on around $1200/fn. So with another child you would get $100 and something extra FTB plus the large family supp although i think thats only 20 odd dollars.
It's still hard when you lose a good half of your entire income to rent but you find a way to make it work as everyone does.
With childcare if you are enitled to PPS you will get the max rate of ccb. With 3 kids i think that is 108%. Also remember if you are working, looking for work or studying you will be entitled to the 50% childcare rebate.