My daughter was 3 and turned 4 shortly after... when I had my 2nd child.
And I had BAD morning sickness AND migraines everyday. AND was so fatigued.
What I did was:
Have a ROUTINE everyday.
I EXPLAINED to my daughter, about my pregnancy... ie: that I would get tired, have a harder time moving around, that Mommy's Doctor said I have to rest etc. and take naps. We would NAP TOGETHER. She understood.
I took her to ALL my prenatal exams and my Doc even taught her how to use the Doppler heart monitor on my tummy.
I used 'my' pregnancy, to FULLY PREP my daughter about her baby brother... in my tummy. We took photos of my growing belly with her, she sang to him, she talked to him, she BONDED with him, already in my tummy.
I TAUGHT her that she can tell me anything. I am there for her. I told her about baby development... that a baby cannot do what she does. He is a baby. That he will cry/I breastfeed/he will wake at all hours. SO that, she KNEW what to expect, once her baby brother came home. I explained, that Daddy will be home with her when I am at the hospital. I made "my" pregnancy, about HER. THUS, once my son was born, she was already... bonded with him, loved him, and was adapted to have a baby, too.
BECAUSE I fully prepped her about it and what a baby is.... before my son even came home.
Thus, she adjusted like a champ.
EVERYDAY, I explained how I was feeling. In a positive way. She would even help me as I went up and down our stairs, holding my hand. I told her dont' worry about Mommy, but as my belly grows, I have a harder time moving and I cannot, carry her. My Doctor told me etc.
You do what you can. And don't do what you can't.
You EXPLAIN to your oldest, about that.
I did not worry, about being "fun" everyday. My daughter, understood clearly, even at that young of an age. Because, I explained to her.
UNLESS you explain to a young child, they will NOT KNOW.... about anything per your pregnancy or about what a baby is once the baby comes home.
That is what I did.
That is what worked for me and my daughter when she was that age and I was pregnant with my 2nd.
You need to prep and explain to your oldest.
I ALSO, fully explained to my oldest when I was pregnant.. .that I DO NOT EXPECT HER TO CHANGE, once her baby brother comes home. That I KNOW, she is a child herself. She can tell me anything. She does not have to 'share' everything either. Her things are hers. It is special. A child needs to know those things. Because, in a little child's mind... those are 'important' things. To know. So that they feel secure.
I told her, Mommy does not expect her to 'act older' or anything. She is herself. Grumpy or happy or needing me... all she has to do is tell me. Even if there is another baby in the home.
Remember: your oldest child is having a 'baby' too. And Mommy will be busy. AND, they will need understanding. They were an only child, for 3-4 years. My kids are 4 years apart.
Once my son was born, my daughter felt secure, because I prepped her for everything... using my pregnancy, for that 9 months, to explain things to her and help her adjust.