I was reading a post on another thread, there are people who have had 2 girls with no problem, it sounds like it becomes more of a problem when you have 2 girls and a single male.
Budgietom makes a good point about how much time you are able to spend with your lone bird. Some people have noticed their birds pining when they lose one of a pair so getting a new companion might be a good thing to do. If you do though don't forget to quarantine! There would be nothing worse than losing 3 in quick sucession. At least your current bird would be able to hear the new addition and know a friend was on the way.
Thanks for the siggy KipTheGreat