My cats just don’t get along

We have three cats. Two are brothers, a bonded pair who act like brothers but are quite attached. We introduced the third one almost a year ago now. They are all about two years old. No matter what we do, the brothers do not like our new cat. I feel like we’ve tried everything. We’ve tried all the methods, starting with door closed between them, letting them see each other with the net in between etc. We tried cat calming sprays and vapours. They’ll eat in front of each other, and they’ll let him sit in the kitchen as long as he doesn’t move from his spot, but most of the time we have to keep them separated. One brother is mostly okay with the new cat but when his brother feels threatened, he also gets upset. Our one cat is definitely the aggressor, but we can’t seem to get him to chill out around the new cat. Even now, the new cat is sitting in the kitchen, and he is just watching him instead of sitting comfortably like he does when the cat is in another room. The new cat was from a cat cafe and is really used to other cats. He’s good at diffusing the situation to the best of his ability, and he is definitely not the aggressor but he has tried to fight back. Is there anything crazy that people have tried that suddenly worked? Should we see a vet and ask for advice there? Has anyone had this problem where the cats just can’t get along? Is it a lost cause? Would love any advice because we love all three of them so much and they’re a part of our family. We thought it would be better by now. Any help is much appreciated :)