Congrats on your new babies!! I have 2 PDs and wish I could own a million more!
Whos cage are they in? Do they have toys? How many hours a day do they get to play out of the cage?
I will say, I think your girl is depressed... in a sense. The other two PDs have a bond. As with my experience...
I got 4 PDs back in 2002... they were picked from a group of about 40. I had no clue of the sexes but my 2 havent ever had babies so they must be the same sex... my brothers 2 havent either so they must be same sex as well. They all lived together in the beginning until he moved away. My brother adopted a third PD shortly after and the new PD bonded with one of his first... this was back in 2003 & no babies so thus same sex. He found that one of his original PDs (chancy) was being singled out while the other two bonded. They would sleep together and chancy would be in the opposite corner. She would also shy away from the food bowl when the other two were eating. She became real "depressed" and lost some weight and just didnt act right anymore. The vet told my brother even though they werent physically hurting her that he should not have all three together... so my brother ended up giving the adopted PD back and thankfully if fixed the problem. I personally thought they wouldnt rebond but im glad I was wrong.
Also, when my brother came to visit once we had both cages & connected them together. The PDs all got along ok out but we found that they werent all sleeping together... my 2 together & his two... and they had been old pals just a yr prior. I dont understand either.. I would like to.. but these are very unique animals. Much time is needed just for them to get use to a new owner and new place. This is my only experience and I think that getting more PDs could be successful but much research & time is needed to introduce these guys. PDs need lots of out of cage time, proper diet, & toys to keep them entertained. I would suggest lots of out of the cage play time with toys, tunnels, things they can climb on and hope that they all bond. If they are just in the cage its more likely one will get excluded at sleepy time. Your outsider needs to build her confidence. She also needs to be content with you as her new owner. She may be bonded with her old owner. From what I read, PDs do not like change of cage, asmosphere, owner, etc. I change toys all the time but their cage is there safe place and they go nuts when I move it to clean it. I say, if your three PDs have been living together for awhile unsuccessfully, I say you need to look into making your outsider more happy by separating her and trying to find another PD for her to bond with or a person with all the time in the world for her.