3 is really hard.
Number 2 was actually a much more laid back, compliant baby and toddler till he turned 3.
It was like a mini Kevin the teenager as the clock struck 12.
I remember this phase was the hardest with my first who is now a delightful 8 yr old. What was hard with him is he had boundless energy, ran everywhere and wouldn't settle to anything or play by himself, up at 5 for the day etc. I lived in the park and soft play. I even took emergency chocolate buttons everywhere for "training" him on the scooter to stop at roads and incase he was dangerously going to run off. (Was later admired for how sensible he was on a scooter!)
Haven't had to do that for number 2, he just listens and does. But he throws huge tantrums if he can't get his way that last for aaaagggggees.
They are a bit like teens; their brains are doing some final organising and they're more aware if the world and themselves in it/ growing independence. But also want to be a baby as they basically still are.
They need a lot of help with emotional regulation.
Pick your battles.
Keep them well topped up with snacks and drinks (meals may be in bits through the day.)
Talk about what's going to happen in the day; i draw pictures on a sheet sometimes. I've also done a week with which days are nursery etc, crossing them off. Didn't work at first but now he likes it.
CBeebies is fine; you can scale it back later.
talk to them about an incident when they're calm "I know you wanted x but sometimes we can't and we just wait till next time. Well done for waiting."
Get in the bath with them after an especially hard day. Seems to help.
Toddler Discipline Without Shame and How to Talk to little Kids so they'll Listen and how to Listen so they'll Talk are two books I found really helpful.
(First one advocates against chocolate buttons but for me it was a health and safety thing!)
Many days are utter shit but you're both learning about each other so put it down to a learning process and the next day will be better!