Discipline advice please??

[Lozzabell]

I have 3 sons aged 4 (5 in May), 3 and 16 months old.
My 4 year old and 3 year old fight all the time. Physically and also shout and bicker.
My 4 year old is also getting into trouble at school for being too rough & play fighting.
Both myself and my husband are lovely people... we rarely argue and when we do it's the odd cross word. Nothing major. We have a very happy loving home. I CONSTANTLY promote kindness. I've always been so gentle and affectionate loving with them... so I don't know why they are like this??
I'm at my wits end with knowing what to do and how to stop this.
Currently they get a telling off (I've tried the softly softly calm approach), and they have to sit in 'time out' for as many minutes as their age.
I talk to my eldest every night, as does his dad (my husband) about his behaviour at home and school etc.
We do such lovely things... we walk in the woods, we explore, we play games, do crafts etc. Yet the moment they are left to have free play - they fight!!!

Any advice or reassurance would be great :(

Maybe get them into sports (esp martial arts) to burn off energy and get their need to fight out of their system in a structured way?

When they start fighting and you separate them, maybe ask them how the situation came about, and brainstorm with them ideas about better ways to handle it than shouting, pushing their brother, snatching things off them, etc? Eg breathe slowly 5 times in and out to help remain calm, then get parent for help?

Another option which worked well for me when I was a kid - if fighting over a toy or other object, take it off them without any words, don't give it back until the next day. If they fight over something too much, it goes in the bin.

I totally agree about the sport, I tend to find that boys need exercising to the point of sheer exhaustion every single day. Lots of love, lots of food and lots of exercise, a bit like labradors

So things like the park, swimming, walks in the woods. For the older one football or Rugby in the winter and tennis or cricket (or both) in the summer,

Calm Parents, Happy Siblings might help too