Birds will react to each other no matter what, and yes...getting another one to live in "your house/territory" is quite unsettling at first..
Appie is still a bit stressed and has been overgrooming to the point of almost plucking ever since Sunny came here.
(Yup, it still freaks me out every single time she spends too much time grooming.)
I have noticed over the years it is Appies way of dealing with tension, but it is somewhat worriesome.
(Sunny moved in over 10 months ago, Appie has been here al least a year before her.)
Keep hoping they will settle down asap- but you cannot hurry nature...
(esp greys who love to study every single aspect of a situation before deciding to be okay/not okay with it and a somewhat insane-in-the-membrane semi-shellshocked macaw who needs a lot of time to decide on her personality)
Japie is quite happy-go-lucky about it, so the first bird here is really helping the other 2 to settle in by not stressing and being chill about being handled etc.. (When he is not divebombing the macaw or Appie "out of the blue". Still no clue what motivates him at those moments...)
But it is an uneasy situation sometimes (uncertainty -> agression), I keep doing flock-things with them: they shower together whenever they want to shower, they get special treats together, if they tolerate each other they have cuddlytime together (groupgrooming) as wel as individual of course.
So I hope it will eventually dawn on them that being a flock is actually fun!
And they are mellowing out a bit, but I think it will take them a long, long time to get really comfy with each other.