Thank you all for your replies, it really helps.
My dad says he doesn’t want to do a necropsy as he says they can be expensive. My plan right now is getting the other three checked out to make sure the same won’t happen to them. Luckily because of the long easter weekend i am able to spend more time with Rio, Angel and Kiwi. They must be down about it too.
One last question; i now have 3, is that a problem? I have heard having an uneven number of birds is not recommended, but to what extent?