except that your oldest is ahead of mine. My 4th has been by far my most high maintenance baby which I was not expecting. It cracks us up when someone will say, "Oh 4th baby. I bet she just goes with the flow." Har-dee-har-har!!! Anyway, here are just a few ideas for what they're worth. My 17 month old still goes in her bed every morning for an hour even though she doesn't actually sleep much during that time anymore. I put a few books, stuffed animals, non-noisy toys in there for her and then we hit school hard for that hour trying to accomplish anything that I must be around for. Now...lots of days she pretty much fusses the entire time(!), but as long as I know she's not hungry or doesn't have a dirty diaper and isn't sick...she just has to stay! I hope that doesn't sound too harsh. She gets so much attention the rest of her waking hours, I just don't worry about it!
As for the preschooler, I must admit my 4 yr. old is really good at entertaining himself, but I have found a few things that help. First of all, I really try to sit with him and read a couple of picture books before we do school. The olders aren't allowed unless he invites them. It's just his own time. He really like this and I try to make it a big deal. 2 or 3 days a week he goes to his room for our hour of time and listens to a CD and plays with a few things in there. The other days he stays in with us, sometimes just playing, sometimes at the table. There are more interruptions those days, but I count that as learning for everyone too! My 2nd grader does any leftover school work after that hour on his own. Obviously if he needs me I'm there, but we really try to get the more dependent things done during that hour.
By the way, naptime is completely untouchable at our house!!! Even my big kids go to their rooms for rest time every day. I avoid errands, activities, etc. like the plague during the afternoon. We actually do almost no errands during the week at all. I go at night or we go on the weekends when dh is home and can go along or keep some/all here.
I'm sorry that got long. I lurk here constantly and rarely post, but when I do I tend to ramble! I hope this is helpful somehow, even if it's just in knowing that the next time your 3 year old is running around and the baby is crying for you, I'm probably listening to/watching the same thing!!! Hope you have a good day today!