I am working with an almost 5-year-old, a 3-year-old, and a 7-month old. I'm also still nursing, but we started cereal/solids sometime in the past month. I do not feel like all the balls are being juggled well, but my kids are happy and healthy. I found that, as suggested, sleep is your salvation here. I do not get enough (I think I get an average of 5, but often more like 4 1/2) and since my husband works evenings I cannot even nap after work and he's away until after 11. If you're not comfortable co-sleeping (in bed) then have the baby in your room as long as possible. That helps with nursing at night and maximizing your sleep. We try to keep ours in his bed most of the time, but some nights I get more sleep as a human pacifier and I need sleep more than I need space in bed.
I also had a "before bed" and "before work" checklist that I made after each of my kids. I actually went through and checked everything off, including things like 'wash breast pump" "make lunch" "set up bottles" and "set out clothes, jewelry, shoes, makeup" as appropriate. In the morning my checklist included nursing the baby, getting the baby back to sleep for daddy, making coffee, etc. I'm so fried that I do better with a piece of paper telling me what to do.
Good luck. You'll do great! I'd give housekeeping advice, but I'm usually in need of it myself. :)