Why are my cats fighting?!

I have 3 cats, 1 female age 8, one Male age 8, and one Male age 2. All of them are spayed/neutered. Both 8 year olds have lived together their whole lives, we recently added in the 2 year old about 6-7 months ago. All of a sudden both males are constantly fighting eachother- leading to small injuries on both ends. They haven't had any issues until about two weeks ago. No environmental changes, no schedule changes, nothing that would've seemed to make this happen suddenly. They just seem to want to piss the other off and it leads to a cat war. they get separated, calm down, are fine for a day and then it starts again. Any ideas? We have the cat pheromone diffuser designed to help but are looking for more ideas!