Despite her getting a clean bill of health by the humane society, you still need to quarantine her from your other chins for at least 30 days. This means separate cage in a totally different room and absolutely no contact until those 30 days are up. While she may seem healthy now, the stress of a move and new home can cause her immune system to weaken and allow any underlying illnesses/parasites to crop up. Other illnesses also have longer incubation periods and she could have been exposed to something but not showing signs of illness yet.
Unless they took x-rays, there's no way to know for sure she isn't pregnant if she's been exposed to a male. Even x-rays can miss the babies if they are taken too early. Gestation is 111 days, I honestly would keep her alone in a baby safe cage for that long if there is any chance she could be pregnant.
Rushing is not a good way to go about introductions. Just because she's healthy doesn't necessarily mean they will by any means get along. In my experience, introducing a new chin to an already bonded pair can go very sour. Instead of getting a trio, the bonded pair may begin to fight because of the upset in dominance so you end up with 3 single chins instead of 3 bonded ones. I have had trios before but they only got along for a few months before starting to fight. I have had to completely separate them, and in another instance I got two paired back up but the third is a single chin. Of course some people have trios that are okay, but I have read many stories on here about trios not working out and in my own experience I have 0% success rate with them. And of course, some chins may never get along with others, may only like certain chins but not others, etc. There's no guarantee the new girl will get along with your other girls, or vice versa.
After the quarantine is over, you still need to take introductions slowly as rushing them usually does more harm than good. I would move their cages side by side for a few weeks until they are comfortable with each other's presence and then slowly go from there.