Trouble with new introduction

Hi guys,
   
I have 2 female guinea pigs 7 and 5 months old. I introduced them with no problems at all, 2 months ago. I have recently adopted a 4 week old female rex who is of much calmer personality. But am super confused by my girls reaction.
   
One minute they are fine, popcorning, eating and the usual uncomfortable bum sniffing. The next minute they are chasing her around and butting into her. Is this just making sure she knows who's boss or does it go beyond this??
   
They 5 month old makes a funny noise, similar but not the same as a snake hissing. I have never heard the teeth chattering or a hiss before so I dont have anything to relate it to.
   
The new girl, is eating, popcorning and running around though somewhat timidly. But she lets out a very loud shriek when the girls get close to her.
   
With the mix in behaviour I am not sure if this is going well or not. Please HELP??

Three constitutes a 'herd' and they are essentially sorting out who the boss hog is.
   
It sounds like the 5 month old is going to win the battle for top gal in the herd.
   
Be sure you have plenty of space for them and keep an eye to be sure they don't 'gang up' on the newbie.
   
Once the 'boss' has been established, be sure that one doesn't hog up all the food. If you notice her pigging the food, then you need to separate her at feeding time to be sure the others get their fill.
   
It sounds like the first two were doing all right together and now their boat has been rocked. Things may never completely settle down but I think since they aren't constantly going at her, it should be all right over time.