Get things ready, the NIGHT BEFORE.
Allow, MORE time than you actually need, in the morning to get yourself AND your kids, ready.
ALLOW for glitches, in your timing, that will get you later.
YOU- get up earlier than the kids. AND get yourself ready first, make breakfast/coffee etc.
Then once the kids wake up, YOU are already ready.
Allow, 1/2 hour to get the kids ready, at least.
So you and them are not rushing.
PUT things in the car already, that you will need, the night BEFORE.
Make their home lunches (if they need that) the night before. Have it in containers in the fridge, already.
Make their water bottles, ready already and filled up, the night before.
Teach your 3 year old, to get ready and do her part too. My son at 3 years old, could get himself ready.
It is all about TEACHING the kids, about ROUTINE. In the morning.
PUT a timer on. Give the eldest a verbal head's up.... ie: We have to go in 10 minutes..... get ready faster....
ALWAYS allow MORE time than you actually need, in the mornings, to get ready. ALL of you. So you are NOT rushing.
AND, think about TRAFFIC. KNOW how long it takes you to drive to school. AND have alternate routes you can take, in the event there is traffic hold ups, on the road.
AND... leave the house, 5 minutes before you actually have to. KEEP in mind, it takes another 5 minutes at least... to load the kids up in the car/buckle them in... and get their things in the car too. And yourself.... before you even have to leave the garage.
THUS, get the kids and stuff, loaded up into the car, 5 minutes BEFORE you actually have to.
Do not wait... until you are down to the wire as far as timing... to load the kids and stuff, into the car.
Have back-up clothing/shoes/diapers, in the car too. At ALL times.
Just in case.
Since your 3 year old is a slow getting-ready child, have her get ready AHEAD of time.... earlier, than now.
AND explain to her, about you cannot be late.
You don't have to have a 'buffet' for breakfast. Just feed her something quick in the mornings. Even a cup of yogurt. Or cereal. Or a breakfast bar. They DO have snack time, at preschool, in the mid-morning anyway.
Make sure, she gets to bed the night before, on time and not late. BECAUSE, an over-tired child in the morning, WILL be cranky and fussier and not able to get ready, on time.