OP, no wonder you're exhausted. You're caring for three kids through a huge upset and putting your needs to one side to do so. I completely understand why, but you can only do that for so long.
You can't do the wonderful job you've been doing without taking time for your own self care. It's not sustainable.
Especially your older two, are they helping out? I think sitting down and saying right, things have changed, we're all in it together, we're all hurting, we can, must and will get through this, might be a good idea. Tell them you need their help - solid practical help. They're capable of more than you think and it will probably help give them back some control over feeling that they have none, if that makes sense.
Lists of chores divided up, and a list of treats each too. Factor in time for yourself, a walk, an hour alone, a hair cut, whatever.
And at some point you're going to need help. Family, friends, child minder, baby sitter. Another list.
Sending you all the very best, hope you have a peaceful night tonight.