Unliveable situation

We have two 6year old cats, who are brothers. We adopted a 10 month old cat in May. We still keep her separated in a large room upstairs because she will fight our two resident cats. She has a lot of room, but I try to let her out every couple of hours. We were working towards introduction veeery slowly because she got loose from her room once and was really agressive towards up one of our resident cats, making him not like her at all. There was fur flying everywhere, yelling...it was very traumatic.

We were making progress with high value snacks with the cats separated from each other through a screen door, but my boyfriend accidentally let the new girl in the living room this afternoon when he forgot she was loose in the hallway and the resident cats were in the living room. A huge fight ensued, which also led to our two resident cats fighting. One of them is now scared and keeps hissing and growling at his brother.

This situation has become unliveable as of now and I am honestly at my wits end...Now not only do we have one cat constantly locked up in a room who needs to be let out every few hours, and needs to be introduced AGAIN...now we also have two brothers who won't get along! I don't know how I can keep the brothers separated? We don't have that much space to keep all of them from fighting and hissing. Please help! (The scared brother also does not like snacks or treats, so we can't reintroduce them through snacks).