If your child is speaking the same language at both home and school:
At 3, while I understand your concerns, I think there is plenty of time to make the transition to school easier. Have you spoken to the daycare? You do not want them pressuring your child to speak.
There are methods that can help reluctant children and the daycare may already know about them or could implement them. These aids include: schedules, pictures and pic symbols, music, asking questions and allowing a yes or no answer -- which may include a nod or head shake. (Example: "Do you want juice or water?")
Child points and selects. "You chose the juice. Here is the juice." When the child gets comfortable, the teacher can make a 'mistake'. The child then has to let the person know that she got water instead of juice. (This is not about denial of what the child needs or wants. It is about soft encouragement to get a message across.)
Every time your child indicates a want or need, the caregiver speaks a sentence back. "You say you need the bathroom. I will take you." "You chose the blocks. Here they are." If there are picture symbols around the room, your child should be encouraged to use them. (These also work for pre-reading so the daycare likely uses them already.) Using pic symbols takes the pressure off speech, but they do not convey shades of meaning -- she can select 'doll' but not that specific doll. When she points to the symbol, it is treated as if she spoke a sentence. The teacher repeats the sentence and then answers. "You chose the doll. Yes, you may have the doll." This in time becomes an opportunity for your child to use her words to select a specific doll.)
If your child is NOT speaking the same language at both home and school: Then I suggest you take language lessons with her to help. Learning a new language is hard and can seem daunting to a little person, even though they pick up languages quickly.
If you truly feel there is something else going on, then I suggest a trip to the doctor. It never hurts to get an opinion and usually they can calm your fears very quickly. All parents worry!