Yes kcladyz and Pretty boy is very right about this subject!
I have 2 budgies and wanted to add 1 more budgie in time, but after a well done research I now understand that uneven budgies in a small flock is a no no! Especially with 2 females and only one boy
. Why its bad with uneven budgies (in general)
1. One of the budgies will always be left out. This birds are highly social flock animals. If one starts to preen its self the others will do the same, they like to synchronize since they are flock animals. If one of the pair starts to preen each other the third one will be left out. And this applies on all the other activities as well. Like feeding each other, playing etc. So even numbers is required.
2. One of the birds will become a target to hack on and bully on!
(If you had more then 5-6 birds it wouldn't matter this much with even numbers btw but 3 is unbalanced)
I don't know how often Bennie and Petra is arguing? but it certainly seems like she is being hacked on? There is definitely some frustration in your group.
So I strongly suggest you to get an other bird to the group in order to create a balanced and happy n content group of budgies ^^ . (but you have to have enough space too in the cage of course, small spaces creates frustrated birds!)