Do either of your boys get on really well with your youngest son??? I would give them their own space, if you can, and get one to share a room with DS3. Maybe put DS3 with DD, and let the boys have their own room, for just a while??

My 11 yr is already waiting for my 2 yr old son to share with him , rather than the 8 yr old.

Maybe they need to be apart more often, need their own space, and not have to fight for it??
Maybe convert some of the garage for awhile?? I think boys esp. need some down time, esp at their ages. Hormones are just a wink away!!

Good luck honey....