She is 3.
You are pregnant.
And, there are TONS of parental "expectations" upon her tiny little shoulders.
YES, a little child can have stress. Of course.
She is 3. And at this age they do not have fully developed emotions yet, nor any coping skills, nor any full mastery of abstract concepts nor the ability to even decipher and analyze their own emotions much less of their parents.
Remember, she is only 3 years old.
4 years old, is also a hard age. Keep that in mind.
When I had my 2nd child, my eldest child, though she adored her baby brother, she had pee accidents. A child's WHOLE world changes, when their Mommy is pregnant and then has another baby.
I've co-slept. And we've also had the mattress on our floor type thing too.
And it was FINE.
When I was a child (older than yours).... I used to go to my parent's room at all hours of the night, to sleep with them. They let me. They knew I would grow out of it. I did. And this is one of my fondest memories of them, of when I was a child.
Your daughter is 3. Look up child development per this age.
It is a hard age... FOR the child.
She is not grown up.
AND yes, all of this is stress, on her tiny little shoulders.
Your daughter has SO MANY expectations upon her and she has to be so perfect.
A little story: my kids are about 4 years apart. While I was preggers with my 2nd, I used that time for prepping my daughter about my pregnancy and about her little brother. Long story short, she adjusted well. She adored her baby brother. She was so mature and wise for her little age. She was not quite, 4 years old yet, but almost. BUT, I always told her that she is not "responsible"... for her baby brother. I am. I am the Mommy. I actually told her I did not want her to get stressed or worried... that she HAS TO "help" with baby. She does NOT have to. That is my, job. Because you see, my daughter WORRIED and did get stressed. She was so young herself, and because, the thought that she had to do SO many things, since she was a "big sister" now, stressed her. And she didn't know what that meant, to be a "big sister." Like how everyone told her. The kid was barely even 4 years old herself. And I never overly repeated to her about how she is a "big sister" and about how she will be a big helper. Because, that is all very pressure filled for tiny young shoulders to carry.
But once, because my daughter was so mature about things, I asked her to do something for me/for her baby brother. But apparently she was unhappy... and she told me in a real emotion filled way "Mommy, I am not a grown up! I'm just a little kid myself!" and she said how "I can't do everything!...." and she was so, not happy.
And it made me sad, that she was putting on such a brave face all this time.... but that, SHE was correct: she was only a little kid herself and not a grown up. And she felt that TOO much, was being assumed about her and about what she was asked to do. And it is the parent's responsibility, to realize that about a little child.
For me, I didn't make my 2nd pregnancy about "me." I made it about prepping my daughter for her baby brother. I didn't tell her how she had to be. But I told her that she can talk to me/tell me her feelings/tell me if she is sad or happy and its okay.
We chatted a lot.
I told her even Mommy may not know something, because having 2 kids is new for me too. So she needs to tell me of whatever she needs.
I told her SHE is my FIRST baby and I will always love her.
BUT I also spent a ton of time with her, even after my son was born.
I didn't want her to be overshadowed.
I ALSO told my daughter, that she DID NOT HAVE TO share everything, nor anything, with her sibling if she does not want to. I told her it is up to her. Her things are special. I understand. So SHE can tell me, what is comfortable for her or not, regarding her things and sharing it.
A little kid, needs to know that. It is a big deal for them.
Many things.
But mainly... "expectations" upon the older child, has to be age-appropriate. A child does not just instantly mature and become perfect and "older" just because they have a baby sibling. They are still just a little kid themselves.
My daughter is 10 now. My son is 6. They adore each other. No jealousy. BUT... recently my daughter was so bent out of shape and frustrated. And she was getting a hissy fit. My Husband was going to scold her. But I looked deeper into my daughter, and I told him NOT to scold her. Sure, she is having a hissy fit... BUT it is because, she is frustrated with her little brother. Because, ALL day, her brother was irking her on purpose and she was trying her best not to let it bother her and she was still being a good "big sister" to him. But now, she FINALLY is past her limit. And it is our son... that needs the scolding, not her.
And I said this in front of my daughter... and when she heard me, say this to my Husband and in front of her brother... she felt so relieved, and she hugged me and cried. Because, she thought no one understood her. She said to us that she was TRYING to be patient with her brother ALL day, but she had no more patience anymore. And at that point my Husband actually APOLOGIZED to her, for not realizing the full situation. He thought, she was merely having a hissy fit for no, reason.
So, the lesson is: once your baby is born... you really need to observe your elder child's cues... and her emotions, to see what is really going on. And talk with her often. It is actually the older child, that will need more time and effort and bonding with you, than the infant.