Sounds pretty normal for a 3 year old!!!! Try some of the suggestions others have given, talk with her about school when you have some one-on-one time. Find out what's really going on from your daughter or the teacher. Don't worry too much, but if it persists for another year or two, recognize this:
It's normal for a child to be concerned about preschool, but not normal for it to consume her throughout the day, nor is it normal when she interrupts enjoyable experiences throughout the day with thoughts and worries about preschool. It's probably not school she hates, but whatever she is worried that relates to school. (see if you can find out exactly WHAT she is worried about, for my daughter it was that she was worried I wouldn't be there to pick her up after that got her stomach in knots. I had been late a couple of times and it had made her extremely nervous that I wouldn't show up at all)
I saw anxiety manifest at around age 4 in my daughter, but didn't really clue in completely till kindergarten. Wish I had some great resources for it. I've tried quite a few things. I'm getting ready for therapy now (age 8). Hang in there. Realize that she is having a very difficult time managing her anxiety in relation to preschool.
I got a book on Amazon about a tomato plant that a child was growing. It was wonderful to have tomatoes for herself and her family. But soon the plant got so big that there were tomatoes everywhere, and she couldn't pick them all, bottle them all or juice them all. They were rotting, etc... Soon all she was doing was taking care of trying to pick tomatoes that there was no more time to play with friends or relax with family. Relate the story to thoughts, how when we let out thoughts grow too big and let them worry us, they affect our lives like a huge tomato plant.
If it persists beyond this school year, explain that it is normal to be worried about preschool, but that we shouldn't think about it all the time. Encourage her to "lock" those worries into a big box in her brain, and we'll discuss it at a certain time together (like bed time or after lunch). See if you can get her to put the thoughts away for a later time and make sure you discuss it with her later.
Recognize that there is probably nothing YOU can say that will take these fears away. Usually having a plan does NOT dispel their worries. They must take control of it, refuse to think about it all the time, and bring it out at a more appropriate time later on to discuss it. Talk as much as you can when it's time to talk. You can roll play coping skills with whatever is her primary concern (act out different scenarios together or with dolls and puppets). Talk about symptoms of worrying, like stomach butterflies, clammy hands, etc... and tell her these are clues to her that she needs to think about something else for a while.
When she is in the situation at school, encourage her to look for another child that might be nervous too, and become friends with them (teachers can help with this). If you're feeling nervous about things at school, there is sure to be other kids feeling the same way too. You can try to help them feel better by being their friend, playing with them, sitting by them, and trying to get them to laugh (laughing is a great tool). Focus her energy on someone else. Definitely talk with the teacher about the anxiety problem, and work with him/her on methods and vocabulary that will help your child cope better.
Fish oil is the best first defense against anxiety in children. Go liquid, as the capsules are too large. I saw some improvement with that, but this is probably a problem that you will have to watch as she grows and matures. You don't want this turning into anorexia or suicide during puberty when hormones and life goes crazy!
Good luck! Right now it sounds totally normal for age 3. If it persists, no worries, she will get through it, because she has a wonderful mother who loves her and is concerned about her!