Should I adopt my children's sibling when I'm struggling with the ones I currently have?

I have three children under the age of three. I have adopted the two older ones and am in the process of adopting the third, who is almost six months old now. I love all of them and really do want to adopt the third child. My only hesitation is that it has been hard these last six months. My oldest who just turned... show more I have three children under the age of three. I have adopted the two older ones and am in the process of adopting the third, who is almost six months old now. I love all of them and really do want to adopt the third child. My only hesitation is that it has been hard these last six months. My oldest who just turned three is very challenging. She is very defiant and throws many tantrums throughout the day especially in public. I keep telling myself that this is just the terrible twos or the terrible threes that she is behaving this way but a part of me worries what if this isn't just the terrible threes and she is going to be a very difficult child to raise. she tested positive for drugs when she was 18 months old, all the children have been exposed to drugs and abuse by their bio parents. It's hard to give the baby the attention I wish I could because I spent every minute of the day chasing after the 18 month old to prevent him from getting hurt, he's in a climbing stage and my three year old is so challenging with her defiance and tantrums. I do have hope that this is just a hard stage of life and that as they get older things will get easier and it won't always be so intense. I do love them all as though they were my own. I would appreciate any insight you have to offer about the future.
Update: I am a single parent.
Update 2: they are all full siblings
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