he's 3.... this age in development is not easy.
Also, in light of his age and the fact that there is a new baby around... keep your "expectations" about him, age-appropriate. If not, there will be frustration on both sides for you and him.
Teach him how to express himself... teach him the names for feelings, good or bad. ie: happy, sad, grumpy, mad, frustrated etc. And teach him that he CAN say it.... and not be scolded for it.
Remember, he is a little kid. Only 3... he is not a big older child. He is 3.
Often, when a 2nd child comes on board... people "suddenly" expect the Eldest child to suddenly 'act' older... but they are not. They are the same... but their world/family has changed. AND, little kids, do not have "coping-skills" nor inherent skills for managing their emotions or abilities. They need guidance.
My daughter was about 3.5-4 when I had my 2nd child. I handled her per her age... warts and all. Imperfections and all. I did not expect her to 'be' older nor to even know what that meant or is. She even once told me, at this young age "Mommy, I'm just a little girl... not a grown up... I need more time..." And she just really taught ME something. So observe your child too... and their abilities or understanding. Do not just go by their numeric age... and 'expectations' based on that or that they are the 'eldest' child.
A kid will say no.... he is speaking up. My son is 3.5 and he does that too... but I am proud of him because he speaks up. But if it is in retaliation... I manage him. Sometimes it is even the parent that is "rushing" a child... and they need more time to do something, not quick like an adult, for example. I also emphasize that we are "family" and we all do 'teamwork." My Son likes that. If he balks... then fine.... unless it is imperative. Or I tell him or my daughter "Redo that.... how can you do/say that better?" And I give them a chance to 'redo' it. Because, this teaches a child OVER TIME... about how to problem-solve and to think of things on their own.... and complete a task. It teaches them a "skill".... not just a punishment.
It is a rock collecting moss... it will not happen overnight... but if given the tools and a child is SHOWN options or alternatives... then they gain SKILLS in HOW to do things or how to think of things.
And often, my kids have taught ME.... other great ways of doing something. And I am proud of them. And I tell them. To me, creative problem solving and coping-skills... .teaches a kid a whole lot more.
Not just an 'absolute' thing.
And sometime too, if a kid is tired, they can't really be on point and perfect... and I don't expect my kids to clean a whole room at those times... I just teach them to help but "try your best".... not about being 'perfect.' And I get better cooperation that way.
Keep expectations age-appropriate.
Keep tasks you want him to do, manageable per his age.
Keep it, within his range of capability and understanding.
He is 3... not yet a 'big-boy' and still a 'baby'... this is a cusp age.... and they are going through TONS of changes.
Pick your battles.
all the best,
Susan