Hi. I definately agree with the post above about the Dwarf Gourami illness issue. Fortunately, the specimens I acquired were QTed and disease-free. I have my M/F pair of Dwarf Gouramis in a 37g with 5 females bettas no problem whatsoever. There may be problems with smaller tanks because of the territory issue.
However, in your case, I would not suggest getting a Dwarf Gourami to replace the third female. That will not help much. In order for female bettas to be able to establish a peck order, you need AT LEAST 3 females; having only two is just asking for trouble. They may be fine together now, but once the betta instincts kick in, both will want to be alpha female and constantly fight until one or both are severely injured or even till death.
Good luck with your decision.