The first t things are this.
1. If the new piggy is younger than four months then she needs to be bonded with them as soon as possible. If she is older than four months then you need to quarantine her for two weeks, in a different room.
2. Introductions should always be done on neutral ground.
Where did you ‘try’ her with the older ones? And where were the rest? What exactly is your older piggy doing to the new one? If she’s being chased and jumped that is all normal.
If you just let her in their side then she may be seen as the intruder. It needs to be on neutral ground with no smells of any pig, a pile of hay in the middle and veg, but no hides.
The problem with just getting pet shop piggies is the fact that your current ones may not bond with the new piggy. In which case you need to have a plan b if things don’t work out. It’s always best, if possible, to take all piggies to a rescue so they can choose themselves their own friend. It’s also sometimes a possibility that they don’t want any piggies joining them. So that’s something to think about in your case.
Another thing is that trios can be hard to maintain, as you often end up with an outsider.
Have a read of the guides I’ve linked below and then see whether you’ll try again. Please make sure you piggy proof it as well. If she can slip through the divider she may be able to slip out of the cage.
https://www.theguineapigforum.co.uk...-stress-fear-and-antisocial-behaviour.167934/
s-behaviour-and-female-health-problems-including-ovarian-cysts.167978/
When Sows Experience A Strong Season (videos)
Bonds In Trouble
Boars: Teenage, Bullying, Fighting, Fall-outs And What Next?
Bonding and Interaction: Illustrated social behaviours and bonding dynamics