Some advice please on settling new Ragdoll

[HardRockOwl]

Posting on behalf of my daughter

She has 2 existing Ragdoll cats - one aged 5 months who is female and one aged about 17 weeks who is male. Both placid as you would expect for the breed. Not from same litter but very bonded

Yesterday she added a third and final Ragdoll to the family. A girl aged 15 weeks.

Now, although fur has not been flying, the existing female especially is clearly not very impressed.

There's been a little bit of hissing and growling (never heard this before!) and she's even hissed at her existing male friend! However they're all still eating, playing and sleeping normally but the occasional hiss and growl.

No attacking, no raised hackles

Is there any advice I can pass on to make the integration smooth?

Thanks in advance

I'm surprised the breeder didn't advise to separate your new kitten for at least two weeks. This is to avoid any infections being carries over, to reduce stress etc.
She needs to separate them and let her settle, slowly introduce each other's scents (on hands, items then swap them around the rooms). If she has a carrier/crate that cats are settled in this can be used to keep all cats safe for first introductions or a glads door when they can see but not touch each other,
Gradual is the key.

As Hobele has said. This is probably the best approach. They just need time to adjust to the idea of each other. Some cats will get along instantly, but others need a little bot of time. Your DD's got her hands full

Jackson Galaxy, the cat man, will have some handy tips in his YouTube vids.