When ds#3 was born, I had an almost 5 year old and a 2 1/2 year old. We basically did nothing that resembled "school" for a while (actually even ds#1's first grade year was very relaxed as ds#3 was very intense and only wanted me to hold him in my arms, 24 hours a day).
This time, I'm due right after Thanksgiving. I'll have a 9 year old, 7 year old, and 4 year old. I have 6 weeks planned of no school (well, I have some independent stuff for the older two to do to keep up skills - math fact sheets, Explode the Code, etc.) so they aren't totally lost when we start again). I'm not sure if that'll be enough (it depends mostly on the temperament of this little bean), but I'm hoping that by then I'll be able to get through at least the core subjects with the older two. At the age of your kiddos, I wouldn't worry about not getting school done right now - maybe when feeding the baby, you can sit and read together a bunch of different books - some literature, some poetry, some non-fiction picture books. Let them draw pictures. Is your oldest writing yet? If so, maybe she can draw some pictures and write about her new little sibling. But for now, keep it simple. It does get better and easier and your older two dds are still quite young in their schooling career.
Congratulations on your newest addition!