3 N-400's - one for your spouse, one for yourself, and one for your 18-year-old daughter.
N-600 for your 15-year old can only be filed after you or your spouse (assuming your spouse is her other parent) complete naturalization. But after you naturalize, you can apply for your and your 15-year-old's US passports together.
90 days before August 16 is May 18, not May 16. But anyway it's already after May 18 so you're OK to apply now. Be careful to check the "resident since" date on your spouse's and daughters' green cards, in case theirs has a different date than yours (it's common for the derivative GCs to be approved a few days after the primary).