S.H.
3 years old is still pretty young to expect her to be completely dry at night, every night.
Most kids do not achieve this until older. My daughter was about 5 years old, when this occurred. No pressuring on our part. It's not necessary.... this is about biological 'readiness' not the child's 'ability.'
Thus, it is a development based... and night-time dryness is COMPLETELY a whole different ability, than day-time toileting.
Just put a waterproof bed pad under her. That is what we do. Then laundering is easier. And she can wear night-time diapers.
But please don't stress her about it... or punish for it. Because it has to do with their body, with which they have no control over sometimes.
At night, my daughter who is 6 years old now, will sometimes have accidents still. No problem. She sleeps so solidly that she simply is not aware of it. She even had a dream once that she WAS on the toilet... until she felt herself get wet. We laughed about it. It's no big deal.
Relax. It's okay. Your daughter is just doing what is appropriate at this young age. Even Kindergarten age children still have accidents at night, if that puts it in perspective for you.
All the best,
Susan