I heard never keep them apart more than 24 hours or they became strangers, but my sources on that came from a website, so that may or may not be accurate.
When I lived with my ex I know he on purpose left the lid of the tank, and I have no idea how long they were missing and running about that room because I was working 2 jobs and was tired one day and didn't get to check in on them (i know that is bad, but alot was going on back then, mentally for me and being tired). Anyhow they got along for awhile, then one day I discoved that the mother had been killed, and then one of the sisters died. But after that they all got along.
I would probably keep the lone one by itself for now, and then introduce it to another gerbil in time. And you may want to put her in another cage or tank on her own. I have just heard that it isn't possible getting a trio together, unless they are very young.
Actually it did work out in one case for me. I had a male, who I thought was a female, and he lived with his sisters and mother for many months, then I found out he was a boy and took him out of there. And he went to go live with his Uncle and cousin, I forget how long it took to introduce them, but it did work out. But then again they were males, females are more tempermental.
But let us know what you decide to do, and I hope it all works out for the best.