I agree that it sounds like there needs to be more time and that they need to sort out who is where in hierarchy.

Our three have a more or less truce with each other. Basically we have 3 cats that do not care for each very much. Although the youngest (and lowest on the totempole) actually would be lost without the other two. She really relies on them for reassurance. The other two are truly both Alpha cats. Basically they seem to have come to some arrangement that they basically just ignore each other.

On the other hand, it is a mistake to even think about bringing another cat into this household. We tried with a stray that adopted us before we moved. Basically all three hate him and decided to hiss and growl if he even moved. He now lives very happily with my mom and my mom's cat who doesn't mind him being there. He's a big cat (about 18lbs when healthy) and is actually an Omega cat. But our three wanted him gone.