OP I'm sorry that you've found yourself becoming the type of parent you don't want to be. It takes practice and I didn't always get it right but I learned a lot from free parenting classes through the local council and taking the elements from my own mams parenting style that I liked and analyzing what I didn't and working out what to do differently.
I decided early on that I didn't want to be a"shouty" mam. We often had discussions from the earliest ages about respecting each other, about how it was to have rules and expectations to keep everyone safe and happy. We were "lucky?" that they had a friend with a shouty dad so we sometimes discussed how that must feel on both sides.
Shouty dad's 9 year old once explained to me that when dad asked them to do something, they knew they didn't have to do it until he shouted, so it seemed that they viewed his asking and repeating as a countdown to the shout which meant the countdown was over. Dad did not see it like that. He felt the children didn't listen to him until he shouted in frustration.
My children and I had an understanding that I would only raise my voice if there was immediate danger and, luckily, the couple of times that I had to, they instantly froze and listened.
Look for some free parenting classes in your area. Are you still seeing a health visitor or similar for your little one? They may be able to make a referral. When things are calm, chat to your older two about how they feel when you shout, let them know in an age appropriate way how it makes you feel. Discuss what you and they can do to work on avoiding shouting and punishments and keeping everyone happier.
Agree achievable targets together with a small reward at the end. Eg: "If we get home from the school run with everyone listening nicely today we can sit and watch an episode of your favorite cartoon together after tea." Act as though you're really looking forward to it. If they don't achieve it, act disappointed about missing out and suggest they try again tomorrow because you'd really like to watch it with them.
Give notice that an activity is ending in 5 minutes, then 2 minutes, etc.
Instead of saying "stop/don't," say what you want them to do. "Walk sensibly beside me please" is a clearer more positive instruction than "stop messing/don't do that".
You can still get back on track. Good luck.