Under normal circumstances I would agree with the comments above. However, in this case you've only had your girls 3 weeks and as the chicken you want to buy will be coming from the same place you may find that they get on well together, as well has having received the same vaccinations so you probably won't have to quarantine them. As Sassy said if you can buy 2 rather than one this may make all the difference.
I lost a chicken to a prolapse within a week of buying them so went back to buy 2 new chickens from the same supplier, put them all in together and had no problem at all.
If you do, just make sure you put out extra feeders and drinkers, just in case.