A decision like this must be very well thought over. You may not know, but it's usually not a very good idea to house two or more females together in the same cage, temperaments can flare up as they get into breeding condition and their hormones go crazy and they start to be overly territorial and aggressive.
Given the fact both your girls get along well and have been living harmoniously, adding a third budgie could mess up the balance. It could be very troublesome to add a third female (unless you are extremely lucky and get a girl with an incredibly sweet temperament).
If you get a male you will run the risk of having both your girls fighting for the male's attention, or in another scenario you will have the male bonding with one of your girls and then the couple turns against your other girl. These are all likely scenarios.
If you do decide to get a companion, then what you can do after quarantine time is over is placing him with one of your girls in a separate cage and your other girl will be alone on a separate cage close by.
Ideally instead of getting one budgie, you could get two males so no bird gets left out without a mate.
Good luck with your decision!