Boxing Gloves - Let the Kids Settle it Themselves
The oldest likes to hit the middle one. She hits back.
The youngest one is putting on weight twice as quick as the other 2 did - he won’t be picked on.
When these guys get older they are going to need a constructive way to address their differences.
My suggestion - Boxing Gloves.
Why might you say? Because they will be able to wail on each other with minimum damage (right now they don’t have any padding and they hit each other).
The only thing is I think my daughter would be the king of the ring 
.