You have to realise that these children were unsure of themselves, and are gradually accepting you as reality.
They are learning to trust you, and are kicking the boundaries to make you respond in a way they are used to. If you don't respond, they will gradually learn to accept you as their primary carer.
However, if their past history has always been to be moved from one family to another, they will expect the same course of action from you - rejection. This is entirely normal, and can only be overcome by earning their trust.
I don't expect this to be easy, nor that you do not go to bed crying, but the children need the help, and love to become rounded adults.
Some of the things you need to learn, are probably how to distract them, and not confront them. Children do not have a long attention span, and you need to break the pattern of confrontation.
Three children who have suddenly been transplaced to your care, need to be active to take their hurt away. Take them for long walks, tire them out and they won't have the energy to confront you - especially if you set the pace!
The eldest has probably been the "mum" to the others, and is kicking against that. She is starting to show that "she doesnt care" when she most desperately cares that someone show her love and the ability to enjoy childhood. She probably needs you to show that you care. This is entirely normal in the adoption of sibling groups.
The treats can come, and they don't need to be much. Helping in the kitchen, or tackling some more responsible task and PRAISING them for doing it well is adequate. Children relish praise, especially children who have been in care. Sanctions do not work.
I wish you well, its a very hard, but it is extremely rewarding task to take three children whose background is extremely sketchy, into your home, and bring them up to be young adults.
So start tomorrow, elicit the help of the eldest to give a good example, to the others. Give them all a treat out walking, and tire them out. Teach them gradually by your own example, but never, NEVER give up.