I would not advise bringing a female into a female situation - in particular when your current females have a 'leader' - basenjis are typically the leader and it will be unpleasant once the jockeying for positions starts.
Now this is going to sound distasteful but please follow along - basenjis are feral and have their own way of dealing with elderly dogs - or infirm dogs. It is unlikely in their natural habitat (Africa) that older dogs were tolerated. While I have seen some nice kind behavior - I have also seen something rather preditory in eliminating an animal that is unable to fend for themselves. While obviously in human care we must not forget that survival of the fittest is still a rule in the wild natural state of basenji.
Your puppy may at first be willing to accomodate the girls (I don't think so but maybe) but at about 1 or so - you will have a problem.
Diana
MIJOKR Basenjis
Since 1983
MN