Your kid is really not going to be in school as much as you think. In addition to summer and 3 weeks of breaks (2 winter, 1 spring), there will also be holidays, snow days and sick days, for all three kids. If you really want to cut costs, then pull the younger two out of school and put them in daycare all day (or choose a school with substantial before/aftercare) and the oldest can be in before/aftercare at ES. Then kiss all your vacation, sick and personal days goodbye because you will be spending them on emergency childcare. Also, you'd better get a jump on the summer camp scramble! It'll be rough with a one-year-old, and you're already late!
Look, having a nanny AND preschool is the most expensive of all childcare options, but you chose that, not your nanny. If my employers told me that they wanted me to work the same number of hours (or wanted me to work a slit schedule with no possibility of pickup work) but were cutting my pay, I would be looking for a new job.