Major changes in her life.
And she is 3.
They don't have "coping skills" to manage their emotions or their lives or their stresses or anxieties.
Hence, they act out.
Wow.... new preschool 1.5 months ago, her Mommy changing and having a baby in 3 weeks, and new big girl bed... all at one time.
Wow.
Kids this age, have no concept of "time" or that is was 1.5 months ago that she was in a new preschool. For example. To her, it is now.
She is her age... but with many 'stresses' in her life now.
She has no coping skills nor understanding, and emotions are not even fully-developed yet, in a 3 year old.
Thus, she is, being her age in the reactions in how she is reacting to... all of these things.
These things, even adults can have a hard time adjusting to.
So even more so, for a mere 3 year old.
3 is a hard age... developmentally. They are not even yet, fully mastered at emotions/social constructs/self-management or self-reliance. And they do not yet have, fully developed "impulse-control" yet, either.
So, they have a hard time.
4 is a harder age, in some respects.
Tell her Teacher... about all the family's changes right now. So that they understand. And about how this is all affecting her.
An eldest sibling... often needs MORE attention now, when a new baby is coming into play. AND once the new baby, comes home.
My daughter was about 4, when her baby brother was born.
I spent more time on her, while pregnant and once her baby brother came home. And I kept things for her, consistent and constant and JUST for her... to be special. I talked with her, OFTEN. So she could confide in me...and tell me her feelings. I told her it is OKAY... to feel anything she feels... that I am THERE for her. I did not want her to feel I would scold her for every little feeling... as I knew she had a TON to adjust to. I had to be her soft place to fall... and for her to know, that I understood SHE was just a little child herself... not all grown up, just because she was now an "older"sibling.
I had to be, her place she could let her hair down... and vent with and commiserate with. So that, she had an outlet... for any worry/feeling she had.
So we were still close, once her baby brother was born and came home.
She was an only child for about 4 years, before her baby brother was born.
I told my daughter, that SHE was my FIRST baby, and always will be.
That made her feel good. And it was the truth.