This was my age gap too.
Encourage the 3 yo to 'help'. They can fetch the wipes, mommy's water, a muslin, and can sing to the baby with you. They can also help with chores like laundry and wiping down cupboard doors, etc. Don't try doing idealistic/ instagrammy activities that result in you spending five minutes on the activity and an hour cleaning up. I'm thinking here about baking and finger painting and anything involving glitter.
However - don't overemphasise the 'big boy' or 'big sister' label. They are so little, and so young, and they have been your baby only recently.
Be patient with toileting and sleeping regressions, horrible though they are. Be patient with the odd tantrum. Talk with your 3 yo about feelings. Give them the vocabulary to express themselves. 3 year olds are complicated little people who can communicate well, but they have simple needs - demonstrations of love, cheerfulness, empathy, and knowing their grown ups are in charge.?
Make time for hugs, singing, reading, dancing, and getting the 3 yo out to enjoy the fresh air no matter how exhausted you feel. Even sitting with the 3 yo and watching kids' TV is better than leaving him or her sitting in the couch alone while you catch up.
The 3 year olds' father needs to be on the same page and needs to be involved around the house and in parenting.