Your poor Dd.
I think all you can do is be calm , firm and resolute. They have to behave for their teachers at school, and teachers aren’t their Mum.
But it must be hard for them, splitting their time between two homes and parents.
Blended families seem very hard on kids, IMO.
They see their Dad living with your Dd full time, whereas he is only with them part time. They may challenge you over not being Mum because they are jealous or insecure that your own Dd is there.
How long ago did your DH leave their Mum? Were they used to having their Dad to themselves before you all moved in together?
They’ve had a series of big changes: Mum and Dad together, Mum and Dad separate but Dad to themselves, Dad in a new household and family.
Kids often act up as their only means of protest.
Really really hard work for you and I wonder what you expected when you took on 3 younger kids only 18m into your relationship. I think you probably have to look at why their behaviour is as it is.