Any person, will have to adapt and transition to something new and different. Adults can be like that too.
Your daughter has known her crib since birth. Her "bed" is something different. An unknown. Thus, apprehension. A normal reaction.
My son: even at 3 years old, he was in his crib. Why? Well he LOVED his crib and slept SO well in it. And he is a tall kid. But he still fit in it. But then one day, he told us, he wanted to be in a bed. So then we did. A mattress "bed" on the floor. And he slept there fine.
He was ready.
The thing is, the main thing is that she gets sleep. So if that is in her crib or in a bed, or in a mattress on the floor, or sleeping on the floor (which my kids did at times and during phases), I didn't mind. They slept. And I did not force them, to be IN a bed.
One day, she will not be in her crib.
You can try talking to her, or making her be in the bed... and convince her, or have "battles" about it. Because, any "change" tweaks a kid.
Or, with her bed, let her keep Loveys in it, and anything else that is comforting for her. My kids, have TONS of stuffed animals they sleep with, IN their bed with them. My daughter calls it her "nest." Fine. It makes their bed comfy and cozy for them. It is childhood. I did that too, when I was a young child. It is what kids do.
Or ask her, how she wants to make her bed???? Or what she wants to keep in her bed, with her, to make it cozy??? It is her bed, after all.
Don't, parlay your anxiety about it, to her, or because you want the crib for your son.
And, will the baby be in the same room as her?
That will be another, "change" for her.
Getting over something (her crib) or anything a child is fond of, can take time. They are losing, what was.
But no kid, sleeps in their crib forever.
So keep that in mind.
It does not have to be, a battle.
There is no rule, that says a kid has to be in a bed, by 3 years old.
To me, the issue is that, the crib is wanted because your son will be put into it. Then it will not be your daughter's anymore.
And for her, you MIGHT have to explain to her, "why" the crib is not "hers" anymore.
Then the other issue you may have is: once you put your son in HER room, to sleep... then what? Will your daughter get a good sleep that way? Will she wake him? What happens when your son wakes during the night or makes noises? Will his noises, wake her? Will she even like... that the baby is in her room? Will she be expected to control her behavior, and how she is in her room... just because the baby brother is in there with her? Will they, inhabit the room, well? Or not? And in her room... she has her own things and toys... and how will that affect the baby? Babies, should not be around certain objects, per safety and choking hazards, etc. So, how will her room need to be changed, in order to accommodate the baby brother? There is a big difference between a 3 year old and a baby.