I have a 4yr, 2yr and 10 week old so a it similar.
RE: dinner - i get DH to do it and/or keep it very simple, pizza & salad, pasta, beans on toast, even a roast (slam a chicken in oven and ready made yorkshire puds, microwave some frozen peas,corn etc).
I didn't co-sleep all night but would have the baby in with me from 4 or 5am onwards.
DH sleeps in spareroom mostly and deals with the older 2 re: nightwakings, mornings, and breakfast.
Housework is minimal, washing off to launderette for now.
Play with older 2 when baby sleeps, take them to playgroups, park to run around etc. Use sling!!!
Feet up or bed when older kids nap!
DH does a 10pm bottle and I have been known to go to bed at 8pm myslef to catch up on much needed sleep.
It's early days - i'm sure you;re doing a great job it's just very overwhelming