I agree with Julia N.
Also, my daughter was 3, when I was pregnant with my 2nd child.
At her age, I could, explain to her that Mommy has to take care of my tummy, that I need to rest, the Doctor said I need to rest, that sometimes I simply cannot run around, etc.
I simply included her into, my pregnancy and explained in simple ways, what 'phases' I will go through as my tummy gets bigger and her baby brother grows in my tummy.
She understood like a champ.
So... when I had to rest or was too fatigued or had a hard time or had Braxton Hicks... I simply, would give her a head's up.
We'd even nap together. She'd like to rub my tummy and talk to her baby brother in my tummy and sing to it etc. Therefore, it was a COOPERATIVE thing, between us- we bonded that way, she bonded with her baby brother already (instead of just after he came home from the hospital), and she had formed a 'relationship' with him, her baby brother already. And it helped me, because since I explained to her... she was understanding.
So, that is what I would recommend. A 3 year old, needs to be explained to, gently/kindly/simply, what Mommy is needing to do too, for her tummy and baby.
My daughter, never felt left out that way and she was very understanding about it all.
Do not over strain yourself.