3 year olds and 1 year olds CANNOT coexist!

Does anyone have any tips for how I can stop the CONSTANT fighting and screaming between my 3 year olds and my 1 year old over toys? I hadn't even had a sip of coffee yet this morning and I felt like I'd lose my mind.
My 3 year old boys have learned to play very well together, save for a little fighting here and there-- which usually quickly resolves. Enter my 1 year old who only wants to play with what they have or do whatever they're doing and it's WWIII.
It's so difficult. We ended up gating off the playroom which worked for only a few hours. It felt wrong. My 3 year olds were having to climb in and out of the playroom which was a safety issue (the room has large sliding doors which if we leave for them to open and close only results in constant pinched fingers) and DS3 was shut out. Yet, it also doesn't seem right that my 3 year olds build nice train tracks and then DS3 comes in and immediately destroys them. DS1 and DS2 act like DS3 is on their turf whihc I don;t like and and I feel like constantly removing him from whatever they're doing is reinforcing that. But, again, if I leave him, he wrecks what they're doing and it's constant screaming.
What did you do? They have their own room, but it's upstairs and the upstairs is not child proofed at all.
Do I get another superyard and put somebody behind it? Do I let them work it out? That doesn't seem right because DS3 doesn't understand enough yet.
Ugh. I'm about to lose my mind.

 

We have various areas of the house that are baby-safe and other areas that are no-baby-allowed. If the girls are doing something and their brother "wrecks" it, I remind them that there are places they can play where he can't get into their stuff or they can wait to do that particular activity when their brother is napping. We talk a lot about how babies are very curious and like to touch things all the time and that's how they learn. We talk about how babies don't always understand that what they're doing wrecks things, so we have to understand for them and/or help teach them. Is it perfectly fair? No. But it balances out pretty well. They usually spend some of their day all playing together and some of it all playing separately.

 

Our house is similar to Rachel's. We have gated off the dining room and if the twins want to read or do puzzles or color they can go in there and have baby free time. They can also go up to their room and play. The upstairs is not really childproofed either, but I've never had any problem with them getting into things they shouldn't (we started letting them go up there by themselves after they turned 3.) Otherwise, if they are playing in the playroom they know that they are expected to share. If DD2 has a toy that they want, I will tell them, find another toy to offer her and she may trade. I find that if she has a train, she just wants any train and doesn't care if it is Thomas or Charlie or someone else. She will usually happily trade with them if they offer her another train. Occasionally we will still have some screaming and fighting, but they do seem to coexist pretty well.