Firstly, you're a pro at babies, you've got one to preschooler age and survived, all things newborn will come flooding back and you'll wonder why you struggled so much when you had only 1.
Remember when you didn't know what that cry meant, how the poppers on the gro did up right, how to eat and comfort the baby? You won't have the worries this time around.
And best of all, waking in the night won't be so traumatic because you've had 3 years of it already, you're used to it!
So, it's the 3yo that's the challenge. But you're a Pro, so you'll be fine. My suggestions:
Buy a wrap sling and google how to tie it, practice with a teddy or something. I wore mine nearly all day for 6 weeks once dd2 got out of the sleepy put me anywhere and I'll doze off stage. I'd pop her in and out whenever she needed comfort or a nap.
Work at making the house child friendly for a self sufficient 3 year old. Books, simple toys, fruit bowl (if trustworthy) accessible so if you're stuck feeding etc it's not the end of the world.
Buy little toys for difficult times, e.g. Stickers or craft things. Some do a busy basket that only comes out when feeding or settling for naps.
Once baby here and before and talking things through whenever possible, quality time over housework.
Expect major regression (our DD 3.5 had a sudden and complete 'inability' to dress/undress herself for quite a while, even putting wellies on suddenly impossible. She also started hitting DH and I, and bit DD once (just curious really I think). Zero tolerance, no allowances. DD also had a lot of 'convenient' toileting accidents when I was busy with newborn. It all passed.
Bigging up big sister, wow DD, baby is looking at you. Why? Cos she thinks you're amazing! You're walking, eating food, whatever mundane thing, she can't do that yet!
Verbalising whenever baby has to wait for big sister, sorry baby you'll have to wait, I'm brushing sisters hair. Etc
Sorry for brain dump. It'll be hard but you'll be fine. My biggest tip is to master a sling though. There are sling meets you can go to for tips and trying different types