Welcome, and congrats on the birds...
This is a very difficult question to answer, as there is no way at all to predict what type of relationship any of the birds are going to develop with each other...Are they DNA tested for gender yet? I suggest that even before you plan on housing 2 of them together that you pay to have their breeder DNA test the 2 that you get, because if you want to house them together, and you get both a male and a female, they may very well mate/breed, and you certainly do not want siblings to inbreed (since they are siblings and they growing-up together, it's likely that a male and a female will be very closely bonded when you get them, and that pretty much always results in inbreeding if you pair two siblings of the opposite sex).
With the addition of a 3rd bird, again this will depend on a lot of things, mainly the sex of all 3. As they are siblings they will likely get all get along...AT FIRST. But let's say you get a male and a female, and then the 3rd bird is a male or a female...so if two of the opposite sex closely bond, the 3rd bird, regardless of sex, could be bullied and injured...So there's just no way to know how this will end-up, and planning on putting 3 of them together in the same cage is not a good idea...though neither is permanently housing a male and a female who are siblings, as it creates an almost certain inbreeding situation...
