We have never had problems with budgies breeding in the same aviary as long as there is no nesting spots available. No nests, no happy huts, or cozy corners etc. We make sure there are plenty of perches, swings and toys available to avoid fighting and aggression. Also if introducing new budgies to each other observe them closely when placing in the same cage to make sure all are eating well and that one isn't being picked on. We have had some birds that absolutely will no accept another male or female into a cage or aviary even a new different cage.

I really can't tell you how to sex a parakeet that the cere is not an obvious coloration. We have a white with red eyes (albino?) budgie that we were not able to sex without DNA testing so I am sorry I can't help you there.

For cage size the bigger the better. The bare minimum for three budgies would be a cage that is wide enough for three budgies to sit on the same perch all with wings extended fully and still not touching. The cage should be wider than tall or at least as wide as tall as they need more room to hopefully fly then to climb (although many will enjoy climbing as well). I guess if your are asking actual inches the minimum would be 30 inches wide with close enough depth and height.