Children who constantly fight with one another not only disrupt the peace of a home but can wear a parent out emotionally and even physically. I see many parents who have given up and just hide from the conflict in a zombie-like zone with the chaos exploding around them. This is very damaging to the parents and to the children. I keep saying "picture what this will look like at ages 15 or 20 or 35. Now you see you can't let this go on if you care what happens in the life of your child. But once this manner of interacting with each other is established in a home, it is hard to stop it.
The first issue you need to address is each child's sense of individual worth. This requires the parent to give eye contact, physical touch and times of focused attention to each child individually. Children often fight with each other because they are competing for love from their parents. Eye contact and physical touch are as easy as catching your child's eye and smiling or winking at them. Notice when they do something right and look them in the eye and nod your head or walk by and pat their shoulder. Focused attention requires a one on one time. This requires attentiveness on your part but if you want the chaos to cease, put out the effort.
Another area that motivates fighting is having no clear jurisdiction. We often erroneously expect children to share everything; their bedroom, their toys, their clothes, their food etc. We think that is being nice but I propose that instead it teaches a child that you expect them to submit to predators and bullies. Giving a child ownership of say a toy allows him to decide if he will give or not. You as an adult may lend your car to a friend for a need but it would be a very different issue if your friend just drove off in your car one day. That's called theft. And when we expect a child to share all of the time they instinctively feel abused by theft. If each toy in the house BELONGS to one child and his or her name is written on the toy and the other children have to ask politely to play with/borrow it, now we have stopped theft. Your intervention will then consist of asking whose name is on the toy and supporting that child's right to choose what he or she will do with it. You can suggest they might work out a barter deal i.e. "one of mine for one day for one of yours". But then you let them work it out with each child having the final say about their personal property.
If one of your children ALWAYS says no to a request then you spend some time privately with that child teaching them how blessings come our way when we learn to give and bless others. You should not force them to share. You would feel violated if you were forced to share your house with an unsavory individual or even if a loved relative assumed they could come and squat at your place forever. But giving brings blessing to us when it is in balance. Even in shared bedrooms try to divide the room up into jurisdictions. You can use bookcases. That way you will know which child is developing good character and which needs more practice projects.
As ownership becomes accepted and each child feels loved by individual attention from the parents then competition will subside and peace will settle in. Children are selfish naturally so there will be an occasional scuffle but reminding them of jurisdiction and discerning whether you have maybe slipped up on spending some focused attention time with each child will settle the issues quickly before you lose your patience or your sanity.