My partner has 3 kids from a previous relationship.
I won't say there names but I call them Child A, Child B and Child C.

Child A - 12 years old girl
Child B - 11 years old girl
Child C - 5 years old boy

Me and her got together about 4 years after her ex ran off with another girl.

Child A can be very nice and can be very pleasant to be with.
Child A is getting so angry and so ungrateful of everything me and my partner do for Child A. Child A deliberately pisses off my partner by shouting at her, hurting her and calling her names. Child A also picks on the other two when she doesn't get her way.

Child B is so happy when you do something for her, no matter how little it is.
Child B is a very slow learner and gets hurt so easily when someone tell her something (e.g. "you are so stupid", "you can't do it").

Child C is fun and happy sometimes.
Child C can't speak properly (e.g. When he tries to say Christmas he actually says "ismas") and me and my partner are trying to sort out why he talks like that. He keeps on crying when he gets hurt. He sometimes he makes it out that he is hurt mo then he actually is (e.g. If he gets pricked by a thorn on his foot he acts like he can't walk for 2 hours) and gets scared so easily by a lot of things. He cries and screams when ever he don't get his way like a 2 year old and he does something wrong and blames it on the other two.

Me and my partner tries to talk to Child A about why she is so angry most of the time but she don't listen.
Me and my partner tries to talk to Child B about not listen to others when they put her down. We try to boost her confidence in her self but when we try for about 4 months all it takes is someone to say "you are so stupid" or "you can't do it because you are thick" to put her back in square 1.
Me and my partner tries to help to Child C but he still acts like a 2 year old by crying and screaming when ever he don't get his way.

What can we do?
Please give separate answer for each child

Please give a separate answer for each child? That sounds like homework to me.

Anyway, I think you all need therapy. The children apparently have issues. You and your partner cannot handle those issues and nothing you've tried has worked.

You need third-party intervention.

You KNOW your partner is insecure - https://www.askmehelpdesk.com/relationships/how-do-get-girls-trust-628419.html

You speak outloud to imaginary people - and hear their answers - https://www.askmehelpdesk.com/emotional-wellbeing/what-wrong-me-give-me-answers-advice-suggestions-630940.html

You need third-party intervention.

We have tried to talk about therapy.
But my partner can be very stubbern about admitting she has a problem (no matter how big or how small)

I'm puzzled that you think you can get answers online to this whole ball of wax called a family. I don't really see much that is very unusual. You both talk to all 3, and that's the most important part, but maybe you yourself might want to not see this as 3 pat questions with 3 pat answers. Each child is developing his and her personality and character, and they are different - what else is new? Are any of them failing or being picked on in school? They sound pretty typical to me. Even the 5 year old with the slight speech impediment, although it could be from how his palate is formed. Wait to see if a teacher in first or second grade thinks he needs to see someone about it. Other than that, congratulations on caring.

To joypulv

Child A is getting picked on by others at school

Child B is struggeling to keep up on english and maths

Child C's First Grade Teacher is getting a Child Therapist in to examin him because it might be a physiological reson why he talks that way

Thank you for the congratulations